fix(coder): converge agent_sessions.session_id FK to SET NULL (P1.5-b follow-up)

The P1.5-b re-key block (cb1846c) re-adds session_id_fkey as ON DELETE
SET NULL, but the whole block is guarded on chat_id_fkey's absence. A DB
already re-keyed to (chat_id, agent) while session_id_fkey was still
ON DELETE CASCADE never re-enters that block, so applySchema leaves it at
'c' forever — diverging from the schema's stated intent, from worktree_id
(already SET NULL), and from the v2.6.3 changelog's own claim that
session_id is informational SET NULL.

Add a standalone confdeltype-guarded block (mirroring the session_worktrees
defang) that flips session_id_fkey CASCADE -> SET NULL independently of the
re-key gate. Idempotent: fires only while the FK is still 'c' — a no-op on a
fresh deploy (already 'n' from the re-key block) and on every re-run. The
live DB was converged by hand with the identical statements; \d
agent_sessions now shows session_id ... ON DELETE SET NULL.

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

apps/coder/src/schema.sql

@@ -205,6 +205,27 @@ DO $$ BEGIN
   END IF;
 END $$;
 
+-- P1.5-b follow-up: converge agent_sessions.session_id FK CASCADE → SET NULL.
+-- The re-key block above re-adds session_id_fkey as SET NULL, but it is guarded on
+-- chat_id_fkey's ABSENCE — so a DB already re-keyed to (chat_id, agent) while
+-- session_id_fkey was still ON DELETE CASCADE never re-enters that block and stays
+-- 'c'. This standalone guard flips it to SET NULL ('n'), matching worktree_id.
+-- Idempotent (mirrors the session_worktrees defang's confdeltype check): only fires
+-- while the FK is still CASCADE — a no-op on a fresh deploy (already 'n' from the
+-- re-key block) and on every re-run thereafter.
+DO $$ BEGIN
+  IF EXISTS (
+    SELECT 1 FROM pg_constraint
+    WHERE conname = 'agent_sessions_session_id_fkey'
+      AND confdeltype = 'c'
+  ) THEN
+    ALTER TABLE agent_sessions ALTER COLUMN session_id DROP NOT NULL;
+    ALTER TABLE agent_sessions DROP CONSTRAINT agent_sessions_session_id_fkey;
+    ALTER TABLE agent_sessions ADD CONSTRAINT agent_sessions_session_id_fkey
+      FOREIGN KEY (session_id) REFERENCES sessions(id) ON DELETE SET NULL;
+  END IF;
+END $$;
+
 -- v2.6: attribution for DiffPanel badges (Phase 1 UX reads this).
 ALTER TABLE pending_changes ADD COLUMN IF NOT EXISTS agent TEXT;