Slice 12: inline command-result toasts

New <toast-host> overlay: components post 'toast' CustomEvents ({message, kind: success|error|info}) and it shows brief, auto-dismissing, bottom-right toasts (errors linger). <repo-menu> (git + coordination actions) and <pr-list> (create/merge) now post friendly success/error toasts instead of alert(); the activity feed still logs everything. Also adds the slice 11 CHANGELOG entry. Verified live: 'Checked for updates' toast shown.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

components/toast-host/toast-host.md

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+# toast-host
+
+## Intent
+A singleton overlay for brief command-result feedback (AGENT.md §6 polish). It
+gives immediate, legible confirmation of what a UI action did — "Published",
+"Merged & cleaned up PR #3", or an error — instead of only the activity feed or a
+browser `alert()`. Any component can post to it without a reference to it.
+
+## Public surface
+- **Tag:** `<toast-host>` (place once, near the end of the page).
+- **Listens:** `toast` on `document` — `detail: { message, kind?, timeout? }`,
+  where `kind` ∈ `success | error | info` (default `info`).
+- **Behavior:** toasts stack bottom-right, animate in, auto-dismiss (errors last
+  longer — 6s vs 3.5s), and dismiss on click. Renders nothing until posted to.
+
+## History
+- 2026-09-20: created — slice 12; inline toasts for menu/PR command results.
+
+## Notes / gotchas
+- Posters build the `CustomEvent` themselves (components are standalone, no shared
+  module); keep the detail shape in sync with this contract.
+- Messages are set via `textContent` (no HTML injection).