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/docs/tui.md

@@ -135,7 +135,7 @@ export default function (pi: ExtensionAPI) {
 
 This component does not display a text cursor, so it does not need `Focusable`. A component containing `Input` or `Editor` must implement `Focusable` and propagate its `focused` value to that child.
 
-Custom components do not report pi as waiting for the user, because `ctx.ui.custom()` also hosts loaders and other non-blocking UI. When an inline component waits for an answer, pass `blocked: { kind: "question", message: "Pick a color" }` so terminals that support [program status](terminal-setup.md#program-status) show pi as blocked while the component is visible. `kind` is `"question"`, `"permission"`, or `"auth"`. Overlays ignore this option.
+Custom components do not report pi as waiting for the user, because `ctx.ui.custom()` also hosts loaders and other non-blocking UI. When a component waits for an answer, pass `blocked: { kind: "question", message: "Pick a color" }` so terminals that support [program status](terminal-setup.md#program-status) show pi as blocked. `kind` is `"question"`, `"permission"`, or `"auth"`. An inline component reports blocked while it is visible: a newer dialog or component shown in its place reports its own status instead. An overlay reports blocked from when it is shown until it calls `done()`, also while it is hidden with `setHidden()` or covered by another overlay. When several blocking prompts are open, the most recently shown one supplies `kind` and `message`.
 
 Pass `overlay: true` to draw above existing content. Overlay options control size, anchors, offsets, margins, and responsive visibility. An overlay handle can change focus or temporarily hide and show the overlay with `setHidden()` while the interaction remains active.