feat: PM-Agent also need to look at PR comments

From Gitea issue #116.

Authored by the PM agent's worker; the merge gate is the approver. Refs #116.

Agent: pm-agent (issue #116, mode revise)

platform/pm-agent/README.md

@@ -169,16 +169,17 @@ the live Gitea — see `decisions/2026-11-07-pm-agent-pr-mentions.md`.
 With it on, the dispatcher runs a second, independent pass each poll. A mention of the bot
 account on a **pull request** makes the PM read that pull request — its description, its
 changed files, its diff, the thread it was mentioned in and the rest of the conversation —
-and **reply in that same thread**. Both places count: the pull request's conversation, and
-inline review threads against a file and a line.
+and **reply where it was mentioned**. Three places count: the pull request's conversation, a
+review's own body (Gitea renders it in the conversation, so the reply goes there), and an
+inline review comment against a file and a line.
 
 ```
   @pm-agent in a PR comment ─▶ discover (notifications, else open PRs)
-  or an inline review thread    │
-                                ├─▶ read /issues/{n}/comments, /pulls/{n}/reviews,
+  or in a review body           │
+  or on a line inline           ├─▶ read /issues/{n}/comments, /pulls/{n}/reviews,
                                 │        .../reviews/{id}/comments
-                                ├─▶ decide: self-authored? already answered? mentioned,
-                                │        or a follow-up in an already-mentioned thread?
+                                ├─▶ decide, per comment: self-authored? already
+                                │        answered? does it name the bot?
                                 ├─▶ assemble the PR + diff + thread, every field fenced
                                 │        and labelled UNTRUSTED
                                 ├─▶ ask the model: `comment` or `decline`, and nothing else
@@ -191,13 +192,37 @@ ask for work in a shape this program would accept. Nothing on the path calls `bu
 The change-authoring half of issue #116 — revise a branch an agent owns, versus open a fresh
 branch and PR against `main` for somebody else's — is a separate issue and a separate PR.
 
-**"The same thread."** Inline review comments have a thread identity in Gitea (comments on
-the same file and diff position are one conversation). Pull-request conversation comments do
-not — they are a flat list — so the issue path's existing convention is reused: the
-conversation is **one** thread, and from the first mention in a thread onward, every later
-non-bot comment in it continues the conversation without a new mention. A comment made
-*before* the mention is not pulled in, and continuation is per thread, not per pull request.
-That is a policy choice, not a Gitea fact; it is flagged on issue #116.
+**An explicit mention is the only trigger.** A comment is answered if and only if it is not
+the bot's own, it spells out `@`-the-bot-account, and no earlier reply of the bot's says it
+was already answered. Nothing else is actionable: a comment with no mention produces **no
+reply and no model call**, however recently the bot was mentioned or replied nearby. So a
+mention is a discrete, self-limiting request — one `@` gets one reply in the place it was
+written, and the rest of the pull request is read as *context* rather than as a queue of
+questions.
+
+That is the operator's decision on issue #116, taken once it was established that pull-request
+conversation comments are a flat list rather than a hierarchy: *"since there is no threads and
+the PR comments are flat, then the pm-agent should be looking for explicit mentions rather than
+a single mention and continued scrapping of comments within the PR in perpetuity"*. It replaced
+an earlier rule where the first mention in a thread armed replies to every later non-bot
+comment in it. It is applied uniformly, including to inline review comments — those *are*
+genuinely threaded in Gitea, but keeping continuation for them alone would be a second rule
+nobody asked for. The fixtures that used to assert continuation are **inverted, not deleted**,
+so the old behaviour cannot creep back unnoticed.
+
+Two practical consequences. A mention is **repeatable**: ask again by tagging again, and each
+tag gets its own reply. And a mention is **not a conversation** — the PM is told to answer as
+if it is the last word it will see, and to ask you to tag it again if it needs something back.
+
+What remains, unavoidably, is that *anyone* who writes the handle gets a reply — the operator's
+answer (3) is "anyone can talk and have the agent work" — and Gitea cannot tell a quoted
+mention (`> @pm-agent what do you think?`) from a fresh one, so re-quoting an old mention in a
+new comment costs one more reply.
+
+**Thread identity is now only about where the reply goes.** Inline review comments are grouped
+by (path, position), which is Gitea's own grouping and is what makes the reply land in the
+thread that was spoken to rather than beside it; conversation comments and review bodies are
+answered in the conversation. None of that is part of the trigger any more.
 
 **Loop prevention.** There is no author gate — anyone who can comment can start a run, which
 is the operator's decision (issue #116, answer 3: *"anyone can talk and have the agent work.
@@ -223,9 +248,13 @@ bot's *own* comments — pasting it into yours does not silence a mention.
 
 The operator asked for "a way to check for '@'s for the bot account instead of scouring the
 entire pull request list and associated threads". That is Gitea's notification list: a
-mention subscribes the mentioned account to the thread, and the subscription then raises a
-notification for **every later comment on it** — which answers discovery and thread
-continuation with one bounded call.
+mention subscribes the mentioned account to the thread, so one bounded call names the pull
+requests worth reading.
+
+It is a **hint, never the decision** — the trigger is re-derived by reading that pull
+request's comments. That split is why the endpoint over-delivering is harmless: the
+subscription keeps raising notifications for later comments on a pull request the bot was
+once mentioned on, and since those comments carry no mention, they resolve to nothing.
 
 **It will be refused on today's token.** `/api/v1/notifications` sits behind Gitea's
 `notification` scope category, and the bot's token is minted