discord_pr: render a reopened PR, and say so when an action is dropped

[bjornars]

Fixes #9. Stacked on #10 — both touch discord.yml, so this targets that branch rather than main.

Consumers have been sending reopened since novem-code/gaia#4009 landed on 2026-08-28. The composer had no branch for it, so it fell to the else and set send=false: a workflow run per reopen, saying nothing, going green. Measured on gaia, same commit two minutes apart — closed → send=success, reopened → send=skipped. Three of its PRs were reopened in one day and the channel still shows all three as closed.

Three hunks, and the middle one is the one I would argue about:

The reopened branch, draft-aware like opened, because a PR can be reopened while still a draft and "please review" about a draft is what that split exists to avoid.

The else now emits a ::warning:: instead of a plain log line. The job still succeeds — a missing message must not fail someone else's pipeline — but the run names the action it dropped. That branch is how this stayed broken for a fortnight after consumers started sending the event, and it will hide the next one identically. If you would rather not have warnings on runs that are working as intended, this hunk is separable; I think the fortnight argues for it.

This repo subscribes to reopened for its own PRs, which it could not usefully do until the composer knew the action.

Verified by extracting the decision chain and running it against every action this file subscribes to plus an unsubscribed one — seven paths including both new draft variants, and the warning firing on locked. Ordering is unchanged for merged PRs: a closed event still falls through to the PR_MERGED check first.