feat(coding-agent): let ctx.ui.custom() overlays report blocked program status (#11851)

Closes #11816.

## Problem

`ctx.ui.custom()` accepted `blocked` only for inline components.
Overlays ignored it, so an extension question shown as an overlay left
pi reporting `working` over OSC 7501. Example: pi-ask-user in its
default overlay mode inside Herdr App never shows the session as blocked
and never notifies.

## Change

- Each overlay opened with `blocked` registers its own program status
source (`extension-overlay:N`) after `showOverlay()` and clears it when
the overlay calls `done()`.
- The status stays while the overlay is hidden with `setHidden()` or
covered by another overlay: the question is still unanswered.
- Separate sources per overlay, so closing one keeps other open
questions, dialogs and login prompts blocked. With several blocking
prompts, the most recently shown one supplies `kind` and `message`.
- Overlays without `blocked` (loaders etc.) register nothing and cannot
clear another overlay's status.
- `/reload` and session switches drop overlays without calling `done()`;
`resetExtensionUI()` now clears their blocked status so pi does not stay
blocked.
- Inline components are unchanged: they report the dialog shown in the
editor slot, so a non-blocking component shown in its place clears the
status until it closes.
- Docs (`docs/tui.md`, `custom()` JSDoc) and changelog updated.

## Testing

- `npm run check`
- `test/interactive-mode-extension-dialogs.test.ts`: new regression test
(two blocking overlays, one non-blocking overlay, hide, close in order)
and one for clearing the status on reset.

pi-ask-user still needs to pass `blocked` for its overlay prompts.

packages/coding-agent/CHANGELOG.md

@@ -12,6 +12,7 @@
 - Anthropic sessions now use message threads: each request sends only the new messages and the server continues from the previous reply, falling back to the full request when the conversation changed (for example after compaction or a tool list change). On by default for the `anthropic` provider on `api.anthropic.com`; set `compat.supportsMessageThreads` in `models.json` to enable it for gateways that forward the `thread` field, or to turn it off. Cache warming does not touch the session's thread ([#11545](https://github.com/earendil-works/pi/pull/11545))
 - `pi.registerMarkdownTransformer()` takes an optional `{ messageTypes }` that selects the Markdown surfaces a transformer runs on. Besides user, assistant, and thinking Markdown, transformers can now opt into `custom` (custom messages and skill blocks), `summary` (branch and compaction summaries), and `changelog` (What's New, `/changelog`, the Extension Updates notice, and the Update Available note). Without options a transformer runs on user, assistant, and thinking Markdown as before. The built-in Mermaid renderer now also draws diagrams in expanded branch and compaction summaries
 - `pi auth login <provider>` logs in to a provider from the shell without starting a session, for example `pi auth login radius`. It signs in with an account in the browser and exits; `--api-key` saves an API key instead, and `--no-browser` uses device code or copy-code sign-in instead. Logging in to Radius also configures the Radius MCP server. `pi auth logout <provider>` removes the stored credentials, like `/logout`
+- `ctx.ui.custom()` overlays now honor the `blocked` option: pi reports blocked program status (OSC 7501) from when the overlay is shown until it calls `done()`, also while it is hidden with `setHidden()`. Closing one overlay keeps other open questions blocked ([#11816](https://github.com/earendil-works/pi/issues/11816))
 
 ### Changed