Support plugins in side-chat composers

[brsbl]

Summary

• Expose composer plugin actions in pre-child, existing-child, and queued-message side-chat composers.
• Add the minimal public side-chat composer scope contract and generated SDK/template declarations.
• Keep inactive, unmounted, removed, and child-switched composer hosts isolated from stale writes.
• Route side-chat plugin status to the visible parent-thread row.

Validation

• /review-loop: all final reviewers returned CORRECT after ownership, row-targeting, and SDK-documentation fixes.
• Full app suite: 251 files, 1,815 tests passed.
• Focused app suites: 3 files, 60 tests passed.
• Full plugin SDK: 9 files, 60 tests passed.
• App/plugin SDK/templates typechecks, app lint (0 errors), app/plugin SDK builds, declaration/template generation checks, and git diff --check passed.
• The external template scaffold assertion passed in 243 seconds during an isolated diagnostic run; its unchanged local 180-second test budget timed out on this loaded host. The equivalent packages and smoke jobs passed in CI.
• Exact-head desktop QA with an external composer plugin verified pre-child and existing-child replacement, one-control Undo and restoration, cancellation and late-result cleanup, visible-parent row status, queued-editor replacement and manual save, and parent-draft isolation.

All required CI checks pass. No release or plugin publication is included.