refactor: codebase audit cleanup — dead code, dedup, module splits

Multi-agent audit + aggressive cleanup across server/web/coder/booterm,
delivered behind a DEFER discipline so none of the in-flight files were
touched. Removes dead code/deps/columns, dedups server + coder helpers,
and splits the oversized modules (tools.ts, opencode-server.ts,
sentinel-summaries, turn.ts, TerminalPane.tsx) behind stable contracts.
Adds 78 parity/unit tests (server 587, coder 323); fixes two latent bugs
(ChatPane queue keys, FileViewerOverlay blank-line parity).

Intended tag: v2.7.12-audit-cleanup.

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

apps/coder/src/schema.sql

@@ -25,7 +25,6 @@ CREATE TABLE IF NOT EXISTS tasks (
   agent TEXT,
   model TEXT,
   execution_path TEXT,
-  worktree_path TEXT,
   cost_tokens INTEGER,
   started_at TIMESTAMPTZ,
   ended_at TIMESTAMPTZ,
@@ -39,9 +38,9 @@ CREATE TABLE IF NOT EXISTS available_agents (
   install_path TEXT,
   version TEXT,
   supports_acp BOOLEAN NOT NULL DEFAULT false,
-  supports_mcp_client BOOLEAN NOT NULL DEFAULT false,
   last_probed_at TIMESTAMPTZ
 );
+ALTER TABLE available_agents DROP COLUMN IF EXISTS supports_mcp_client;
 
 -- v2.0.0 Phase 4: link tasks to their inference sessions.
 ALTER TABLE tasks ADD COLUMN IF NOT EXISTS session_id UUID REFERENCES sessions(id);
@@ -74,31 +73,10 @@ ALTER TABLE available_agents ADD COLUMN IF NOT EXISTS commands JSONB DEFAULT '[]
 -- v2.2.0: Paseo-style session config on tasks.
 ALTER TABLE tasks ADD COLUMN IF NOT EXISTS mode_id TEXT;
 ALTER TABLE tasks ADD COLUMN IF NOT EXISTS thinking_option_id TEXT;
-ALTER TABLE tasks ADD COLUMN IF NOT EXISTS feature_values JSONB;
-
--- v2.6: one shared worktree per session (all agents/panes in the session operate in it).
-CREATE TABLE IF NOT EXISTS session_worktrees (
-  session_id UUID PRIMARY KEY REFERENCES sessions(id) ON DELETE CASCADE,
-  worktree_path TEXT NOT NULL,
-  base_commit TEXT,
-  created_at TIMESTAMPTZ NOT NULL DEFAULT clock_timestamp()
-);
--- P1.5-b: DEFANG the CASCADE — a session delete must no longer wipe its worktree
--- row. This table is SUPERSEDED by `worktrees` below; all readers are repointed
--- this phase, so the row just persists (dead) on session delete until a later
--- cleanup drops the table. session_id is this table's PRIMARY KEY, so it cannot be
--- nullable → SET NULL is invalid and NO ACTION/RESTRICT would block deletes; the
--- only valid defang is to drop the FK with no replacement. Idempotent: only fires
--- while the FK is still ON DELETE CASCADE ('c').
-DO $$ BEGIN
-  IF EXISTS (
-    SELECT 1 FROM pg_constraint
-    WHERE conname = 'session_worktrees_session_id_fkey'
-      AND confdeltype = 'c'
-  ) THEN
-    ALTER TABLE session_worktrees DROP CONSTRAINT session_worktrees_session_id_fkey;
-  END IF;
-END $$;
+-- tasks.feature_values and tasks.worktree_path were never read or written by any
+-- code path; drop them from existing DBs (fresh DBs never had them in the CREATE).
+ALTER TABLE tasks DROP COLUMN IF EXISTS feature_values;
+ALTER TABLE tasks DROP COLUMN IF EXISTS worktree_path;
 
 -- v2.6: one backend session per (session, agent); resumed on switch-back.
 CREATE TABLE IF NOT EXISTS agent_sessions (
@@ -168,12 +146,9 @@ CREATE TABLE IF NOT EXISTS worktrees (
 );
 CREATE UNIQUE INDEX IF NOT EXISTS worktrees_active_path_uidx ON worktrees(path) WHERE status='active';
 
--- Migrate any surviving session_worktrees rows → worktrees (idempotent; 0 rows
--- after the test-session delete, kept for generality / fresh-DB safety).
-INSERT INTO worktrees (session_id, path, branch, base_commit, status)
-SELECT sw.session_id, sw.worktree_path, 'session-' || sw.session_id, sw.base_commit, 'active'
-FROM session_worktrees sw
-WHERE NOT EXISTS (SELECT 1 FROM worktrees w WHERE w.session_id = sw.session_id AND w.status='active');
+-- session_worktrees was superseded by worktrees (v2.6/P1.5-b); all rows migrated
+-- before P2 cleanup. Drop the dead table; no-op on fresh DBs that never had it.
+DROP TABLE IF EXISTS session_worktrees;
 
 -- Dispatch hint: which chat (tab) a task belongs to. The coder message route and
 -- skills route set it from the frontend tab; session-less creators (arena, MCP,