feat(§8.12/§8.3): main-view Ask cuts an edit branch and runs the question there (v0.54.0)

The AI "Ask" affordance (selection tooltip + prompt bar) had no response
surface on an entry's canonical `main` view: RFCView renders the human-
discussion panel there, not the chat panel, so a chat turn posted from the
reading view had nowhere to render — asking-while-reading silently did
nothing. AI chat is an editing activity (a turn can emit <change> proposals)
and only runs on an edit branch.

Option B: when Ask is invoked from main, transparently cut an edit branch via
the same dispatch as Start Contributing (promote-to-branch for active,
start-edit-branch for super-draft; idempotent — reuse an existing edit
branch), navigate onto it so the chat panel mounts, and run the question
(text + selected quote) as the branch's first chat turn once its view and
main_thread_id resolve. The turn fires from the message-load effect's
continuation (not a parallel effect) so a late message load can't clobber the
optimistic turn; a live ref keeps the latest submitChatTurn closure in reach.

Degrades gracefully: signed-out → login; a viewer who can't contribute has the
cut rejected server-side and the error surfaced (no spurious branch); asking
from a branch already, and Flag on main (human discussion thread), unchanged.

Frontend-only; the branch chat endpoints already work on a branch. SPEC §8.3
records the behavior; new RFCView unit tests + an e2e spec cover the flow.
backend 677 / frontend 66 / e2e 5 green.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

SPEC.md

@@ -1022,6 +1022,20 @@ the user on it in contribute mode. New-branch naming defaults to an
 auto-generated value (user-renamable); the exact format is an
 implementation detail.
 
+The AI chat is an editing activity — a turn can emit `<change>`
+proposals — so it runs on an edit branch, not on read-only main, whose
+right column is the human-discussion surface (§8.12 is branch-scoped).
+Invoking the AI **Ask** affordance (the selection tooltip's prompt, or
+the prompt bar) from main therefore transparently cuts an edit branch
+via the same dispatch as "Start Contributing" (promote-to-branch for an
+active RFC, start-edit-branch for a super-draft per §9.5; idempotent, so
+an existing edit branch is reused rather than a second one cut), lands
+the user on it, and runs the question — text plus any selected quote —
+as the branch's first chat turn. A viewer who cannot contribute has the
+branch cut rejected and the error surfaced (no branch is created); a
+signed-out viewer keeps the §8.7 read-only path. **Flag** from main is
+unaffected — it opens a human discussion thread on main, not a branch.
+
 Discuss vs. contribute is an *intent* affordance, not a *permission*
 affordance. A user without contribute access to a branch sees the
 toggle disabled, with a sign-in or request-access path (see §8.7).