fix(server): persist close_intent on issue↔PR link rows (MUL-2680)

The first take of MUL-2680 gated auto-advance on `closingIdents[id]` from
the current webhook event. That broke the multi-PR sibling case: a PR
declaring `Closes MUL-X` could merge first while a link-only sibling
stayed open, leaving the issue in_progress; when the sibling closed
later, its webhook carried no closing keyword and the handler skipped
re-evaluation, so the issue stayed stuck forever.

Move close intent from per-event state to per-link state:

- New `close_intent` column on `issue_pull_request` (migration 109),
  set monotonically — `LinkIssueToPullRequest` ORs the existing flag with
  the incoming one so a subsequent webhook re-fire without the keyword
  cannot clear it.
- New `GetIssuePullRequestCloseAggregate` query returns open-count and
  merged-with-close-intent-count for an issue. The auto-advance gate
  now reads from this persisted aggregate, which is event-agnostic: any
  terminal linked-PR event re-evaluates and the verdict only depends on
  accumulated DB state.
- Webhook handler links all mentioned identifiers first (writing
  close_intent for the ones declared with a keyword), then iterates the
  affected issues in a separate pass to re-evaluate. The 'only fires for
  keyword-declared identifiers in this event' gate is gone — replaced by
  `merged_with_close_intent_count > 0` against the link rows.

Regression test `TestWebhook_LinkOnlySiblingMergeAfterCloseKeywordPR`
walks the full open→merge→open→merge sequence Elon described and asserts
the issue advances on the link-only sibling's merge.

MUL-2680

Co-authored-by: multica-agent <github@multica.ai>

server/pkg/db/queries/github.sql

@@ -151,20 +151,21 @@ ORDER BY pr.pr_created_at DESC;
 SELECT issue_id FROM issue_pull_request
 WHERE pull_request_id = $1;
 
--- name: GetSiblingPullRequestStateCountsForIssue :one
--- Returns, for the PRs linked to an issue excluding one PR by id (the PR
--- currently being processed by the webhook handler), how many are still in
--- flight (open or draft) and how many have already merged. The webhook
--- handler combines these with the current event's state to decide whether
--- to auto-advance the issue: the issue moves to done only when there is no
--- in-flight sibling AND at least one linked PR (current or sibling) merged.
+-- name: GetIssuePullRequestCloseAggregate :one
+-- Aggregates the issue's linked PRs into the two counts that gate
+-- auto-advance: how many are still in flight (`open` or `draft`) and how
+-- many merged PRs declared explicit closing intent on the link row. The
+-- webhook auto-advances the issue when open_count = 0 AND
+-- merged_with_close_intent_count > 0. Both the PR state and the link row
+-- (with close_intent) are persisted before this query runs, so the result
+-- is event-agnostic — a link-only sibling closing after a closing-keyword
+-- PR has already merged still resolves the issue.
 SELECT
     COALESCE(SUM(CASE WHEN pr.state IN ('open', 'draft') THEN 1 ELSE 0 END), 0)::bigint AS open_count,
-    COALESCE(SUM(CASE WHEN pr.state = 'merged' THEN 1 ELSE 0 END), 0)::bigint AS merged_count
+    COALESCE(SUM(CASE WHEN pr.state = 'merged' AND ipr.close_intent THEN 1 ELSE 0 END), 0)::bigint AS merged_with_close_intent_count
 FROM github_pull_request pr
 JOIN issue_pull_request ipr ON ipr.pull_request_id = pr.id
-WHERE ipr.issue_id = $1
-  AND pr.id <> $2;
+WHERE ipr.issue_id = $1;
 
 -- =====================
 -- GitHub PR check suite
@@ -195,12 +196,19 @@ WHERE EXCLUDED.updated_at >= github_pull_request_check_suite.updated_at;
 -- =====================
 
 -- name: LinkIssueToPullRequest :exec
+-- close_intent is monotonic: once a PR has been linked with closing intent
+-- (a "Closes/Fixes/Resolves" keyword in title or body), a later webhook
+-- re-fire for the same PR without the keyword (e.g. body was edited to
+-- drop the keyword after merge) must not clear the flag. The OR-merge in
+-- DO UPDATE keeps the strongest declaration that has ever applied to this
+-- link.
 INSERT INTO issue_pull_request (
-    issue_id, pull_request_id, linked_by_type, linked_by_id
+    issue_id, pull_request_id, linked_by_type, linked_by_id, close_intent
 ) VALUES (
-    $1, $2, sqlc.narg('linked_by_type'), sqlc.narg('linked_by_id')
+    $1, $2, sqlc.narg('linked_by_type'), sqlc.narg('linked_by_id'), $3
 )
-ON CONFLICT (issue_id, pull_request_id) DO NOTHING;
+ON CONFLICT (issue_id, pull_request_id) DO UPDATE SET
+    close_intent = issue_pull_request.close_intent OR EXCLUDED.close_intent;
 
 -- name: UnlinkIssueFromPullRequest :exec
 DELETE FROM issue_pull_request