fix(durable): rank tool search like the coding agent (#11631)

Ports the coding agent's tool ranking fixes for #11331 (#11440, #11625)
to durable, which keeps its own copy of the ranker in `src/search.ts`.

Problem: durable's `rankDocuments()` was still the coding agent's ranker
from before both fixes. `thread_read` with `limit: 1` returned
`queue_read`, and a tool name among other words (`peekaboo app`) lost to
tools that only mention the words in their descriptions.

Change:
- `src/search.ts`: same scoring as the coding agent's `Bm25Ranker`, plus
coverage first: a tool matching more of the query's terms (in any field)
ranks above one matching fewer, whatever the scores. Without it, name
weighting regressed `screenshot window` (1 to 2, `window` beat `image`)
and `describe app` (1 to 2, `app` beat `see`). The name is a separate
field (3x IDF per term, above any number of description mentions), a
tool whose whole local name is in the query gets a bonus worth one more
match of every query term, a query that is exactly a tool's name ranks
it first, and namespace text is a separate 0.5x field that is not
length-normalized.
- `tool_search`: the search text no longer repeats the name; the ranker
reads it from `name`.
- `searchTools()`: same, and the namespace text now includes the
namespace instructions, as in the coding agent.
- `test/search.test.ts`: the coding agent's ranker cases.

Remaining difference from the coding agent: durable's `tool_search` has
no namespaces (they are a code mode option), and `searchTools()` ranks
on the tool sample from the start data (description plus TypeScript
declaration) rather than description plus schema text.

`searchTools()` results are journaled, so replaying a script started
before this change returns the old results; no `CODEMODE_MODULE_VERSION`
bump.


## Before/after

`searchTools()` ranking on a made-up set of 16 MCP tools from 5 servers.
Each tool is searched with its description; each server is a code mode
namespace (prefix `mcp__<server>__`) with the description and
instructions below. Rank of the wanted tool among all matches, with the
top result when it is another tool.

| Server | Description | Instructions | Tools: description |
|---|---|---|---|
| `github` | GitHub repositories, issues, and pull requests | Use
owner/repo for repositories. Issues and pull requests share numbers. |
`list_issues`: List issues in a repository, filtered by state, labels,
or assignee.<br>`get_issue`: Get an issue with its comments by
number.<br>`create_issue`: Open a new issue in a
repository.<br>`search_code`: Search code across
repositories.<br>`create_pull_request`: Open a pull request from a
branch. |
| `linear` | Linear issue tracker | Search issues before creating new
ones. Issues belong to teams. | `search_issues`: Search Linear issues by
text.<br>`create_issue`: Create a Linear issue in a team. |
| `peekaboo` | macOS automation | Peekaboo controls macOS apps. Use app
names or bundle IDs. Peekaboo can capture any app window. | `app`:
Launch, quit, relaunch, focus, hide, or switch applications. Targets an
application by name, bundle identifier, or process ID, and can wait
until it is ready.<br>`see`: Capture the app window and describe the UI
elements of the app.<br>`window`: Move, resize, or close a window of an
app.<br>`menu`: Click a menu item in an app.<br>`image`: Take a
screenshot of the screen, an app, or a window. |
| `app` | Thread and queue state of the app | Threads hold messages; the
queue holds pending messages of a thread. | `queue_read`: Read a queued
message of this thread.<br>`thread_read`: Read durable state and a
paginated timeline from any thread. The default view returns user
messages, assistant messages and plans. Continue with afterPosition
until hasMore is false. |
| `docs` | Project documentation | Search the docs before answering
questions about the API. | `search`: Search the
documentation.<br>`read_page`: Read a documentation page by path. |

Before, a tool's search text was its name, the name with `_` as spaces,
its description, and the namespace name and description (no
instructions). After, the name and the namespace (with instructions) are
separate fields.

| Query | Wanted | Before: rank | After: rank | Change |
|---|---|---|---|---|
| `thread_read` | `app__thread_read` | 2 (top: `app__queue_read`) | 1 |
$\color{green}{\textsf{better}}$ |
| `mcp__app__thread_read` | `app__thread_read` | 2 (top:
`app__queue_read`) | 1 | $\color{green}{\textsf{better}}$ |
| `peekaboo app` | `peekaboo__app` | 5 (top: `peekaboo__see`) | 1 |
$\color{green}{\textsf{better}}$ |
| `app` | `peekaboo__app` | 4 (top: `app__queue_read`) | 1 |
$\color{green}{\textsf{better}}$ |
| `read thread messages` | `app__thread_read` | 2 (top:
`app__queue_read`) | 1 | $\color{green}{\textsf{better}}$ |
| `open the peekaboo app tool` | `peekaboo__app` | 5 (top:
`peekaboo__see`) | 1 | $\color{green}{\textsf{better}}$ |
| `launch app` | `peekaboo__app` | 1 | 1 | $\color{gray}{\textsf{same}}$
|
| `create linear issue` | `linear__create_issue` | 1 | 1 |
$\color{gray}{\textsf{same}}$ |
| `search documentation` | `docs__search` | 1 | 1 |
$\color{gray}{\textsf{same}}$ |
| `read docs` | `docs__read_page` | 1 | 1 |
$\color{gray}{\textsf{same}}$ |
| `pull request` | `github__create_pull_request` | 1 | 1 |
$\color{gray}{\textsf{same}}$ |
| `get issue comments` | `github__get_issue` | 1 | 1 |
$\color{gray}{\textsf{same}}$ |
| `screenshot window` | `peekaboo__image` | 1 | 1 |
$\color{gray}{\textsf{same}}$ |
| `describe app` | `peekaboo__see` | 1 | 1 |
$\color{gray}{\textsf{same}}$ |
| `create issue` | `github__create_issue` | 2 (top:
`linear__create_issue`) | 2 (top: `linear__create_issue`) |
$\color{gray}{\textsf{same}}$ |
| `search github issues` | `github__list_issues` | 5 (top:
`github__search_code`) | 5 (top: `github__search_code`) |
$\color{gray}{\textsf{same}}$ |

No regressions on this set. `search github issues` stays at 5:
`list_issues` does not contain "search", so it ties on coverage (2 of 3
terms) with `search_code` and `linear__search_issues`, and their scores
win.

packages/durable/docs/spec.md

@@ -3808,8 +3808,14 @@ declares them, built from the resolved tools. Loaded tools follow the other
 tools in load order, so a load appends declarations to the transcript and the
 cached prefix holds. `@earendil-works/pi-durable/tool-search` provides the
 coding agent's `tool_search` (`createToolSearchExtension()`): it ranks
-`Agent.deferred` with BM25 over the tools' names, descriptions, and parameter
-names and descriptions, and loads the matches. The `codemode` section does not
+`Agent.deferred` with the coding agent's tool ranking and loads the matches.
+The ranking is BM25 over the tools' descriptions and parameter names and
+descriptions, with the name as a separate field: a tool matching more of the
+query's terms ranks above one matching fewer, a query term in the name
+outweighs any number of mentions in the description, a tool whose whole name
+after its last `__` is in the query gets a large bonus, and a query that is
+exactly a tool's name ranks it first. Code mode's `searchTools()` ranks the
+same way, with each tool's namespace text as a further, low-weight field. The `codemode` section does not
 list deferred tools; scripts find them with `searchTools()`.
 
 Tools reach files and processes only through `api.env`, never through an