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)

runbooks/pm-agent.md

@@ -22,9 +22,9 @@ In order of increasing severity. Each is an operator action.
 and creates no worker Jobs. This is also the right setting for watching it for a day.
 
 `PM_DRY_RUN` also silences the **pull-request mention** path completely — that path only ever
-posts a comment, so leaving it posting would make a dry run not dry, and its reply doubles as
-the marker that changes what it does next. To silence just that path and leave issue triage
-alone, set `PM_PR_MENTIONS: "false"` instead (that is the default; see
+posts a comment, so leaving it posting would make a dry run not dry, and its reply carries the
+marker that stops the same mention being answered again. To silence just that path and leave
+issue triage alone, set `PM_PR_MENTIONS: "false"` instead (that is the default; see
 `platform/pm-agent/README.md`, "Mentions on a pull request").
 
 **2. Stop it immediately, without a merge.**
@@ -96,11 +96,17 @@ it to look again, comment on the issue.
 **It will not re-answer an issue within `PM_ISSUE_COOLDOWN_SECONDS`** (15 min) of its last
 attempt on that issue. Restarting the pod clears that timer.
 
+**On a pull request it answers explicit mentions and nothing else.** A comment that does not
+write `@`-the-bot-account is never answered and costs no model call, however recently the bot
+was mentioned or replied on that pull request. If you asked something and got no reply, the
+likely cause is a missing `@` — tag it again. (Near misses do not count either:
+`@pm-agent-worker` and `[email protected]` are deliberately not mentions.)
+
 **It will not answer the same pull-request comment twice.** Every reply it posts on a pull
 request carries a hidden marker naming the comment ids it answered
 (`<!-- pm-agent:replied-to=… -->`), and it skips anything already marked. To make it answer
-again, write a *new* comment — editing the old one does not change its id.
-`PM_PR_COOLDOWN_SECONDS` (15 min) bounds it to one decision per pull request per window.
+again, write a *new* comment that mentions it again — editing the old one does not change its
+id. `PM_PR_COOLDOWN_SECONDS` (15 min) bounds it to one decision per pull request per window.
 
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