feat(hooks/script): integrate script handler — FireResult + source-tier gate + migration

Wave 1 Phase 03. Wires the Phase 02 Goja handler into the dispatcher, widens
the DB schema for script + builtin sources, and refactors Dispatcher.Fire to
return a FireResult so builtin-source hooks can mutate event input.

Dispatcher:
- FireResult{Decision, UpdatedToolInput, UpdatedRawInput} replaces the plain
  Decision return. stdDispatcher.runSync keeps a local evMut copy so a
  builtin-source hook's updatedInput flows to downstream hooks in the chain
  and to the caller; non-builtin (source=ui) script mutations are dropped +
  WARN-logged (defense-in-depth; source tier enforced at the dispatcher, not
  only at the handler).
- applyBuiltinMutation + placeholder builtinAllowlistFor (rawInput, toolInput)
  — Phase 04 overrides the allowlist from builtins.yaml.
- noopDispatcher returns FireResult{DecisionAllow}.
- Script mutation tests: TestDispatcher_ScriptMutation_BuiltinSourceApplies,
  TestDispatcher_ScriptMutation_UISourceDenied.

Pipeline + callers (14 Fire sites refactored):
- FireHook wrapper returns FireResult; context_stage.go:52 applies
  UpdatedRawInput → state.Input.Message; tool_stage.go:52 applies
  UpdatedToolInput → tc.Arguments before ExecuteToolCall.
- delegate_bridge + delegate_tool keep the Decision branch; Updated* ignored.
- dispatcher_test / delegate_bridge_test / delegate_tool_hooks_test /
  integration test helpers updated for the new shape.

Validation:
- edition_gate allows HandlerScript on every edition (sandboxed, no shell
  escape surface).
- config.validateHandler rejects empty source, source > 32 KiB, goja compile
  error; validateTimeout rejects on_timeout=ask|defer (reserved).
- Six new config tests cover the script path.

Migrations:
- PG migration 000053 relaxes handler_type + source CHECKs and drops
  uq_hooks_{global,tenant,agent} — scripts routinely want many small hooks
  per event. RequiredSchemaVersion → 53.
- SQLite SchemaVersion → 21; patch 20 is a SELECT 1 placeholder because
  SQLite can't ALTER a CHECK — rebuildAgentHooksV21 runs outside the
  migration tx (parallel to backfillV16) to rename/recreate the table with
  widened CHECKs. schema.sql fresh-DB path matches.
- H9 fix: PGHookStore.Create + SqliteHookStore.Create honor caller-provided
  cfg.ID (uuid.Nil falls back to UUIDv7), unblocking Phase 04 idempotent
  UUIDv5 seed. TestCreateHonorsFixedID on both stores.

Config:
- config.HooksConfig{ScriptConcurrency, ScriptPerTenantConcurrency,
  ScriptCacheSize, BuiltinDisable}; buildHookHandlers wires the script
  handler with the caps from appCfg.Hooks.

Gates green: go build ./... + sqliteonly, go test -race -count=1 across
hooks/pipeline/sqlitestore.

internal/store/sqlitestore/schema.sql

@@ -1570,9 +1570,12 @@ CREATE INDEX IF NOT EXISTS idx_vault_links_source
     WHERE json_extract(metadata, '$.source') IS NOT NULL;
 
 -- ============================================================
--- Table: agent_hooks (migration 000052)
+-- Table: agent_hooks (migrations 000052 + 000053)
 -- SQLite translation: JSONB→TEXT, TIMESTAMPTZ→TEXT, UUID→TEXT,
 -- BYTEA→BLOB, DATE→TEXT (ISO8601), CHECK for enums.
+-- v21 (migration 000053): handler_type adds 'script', source adds 'builtin',
+-- old (event, handler_type) uniqueness indexes dropped — scripts routinely
+-- want many small hooks per event.
 -- ============================================================
 
 CREATE TABLE IF NOT EXISTS agent_hooks (
@@ -1581,7 +1584,7 @@ CREATE TABLE IF NOT EXISTS agent_hooks (
     agent_id     TEXT REFERENCES agents(id) ON DELETE CASCADE,
     scope        TEXT NOT NULL CHECK (scope IN ('global', 'tenant', 'agent')),
     event        TEXT NOT NULL,
-    handler_type TEXT NOT NULL CHECK (handler_type IN ('command', 'http', 'prompt')),
+    handler_type TEXT NOT NULL CHECK (handler_type IN ('command', 'http', 'prompt', 'script')),
     config       TEXT NOT NULL DEFAULT '{}',
     matcher      TEXT,
     if_expr      TEXT,
@@ -1590,25 +1593,13 @@ CREATE TABLE IF NOT EXISTS agent_hooks (
     priority     INTEGER NOT NULL DEFAULT 0,
     enabled      INTEGER NOT NULL DEFAULT 1,
     version      INTEGER NOT NULL DEFAULT 1,
-    source       TEXT NOT NULL DEFAULT 'ui' CHECK (source IN ('ui', 'api', 'seed')),
+    source       TEXT NOT NULL DEFAULT 'ui' CHECK (source IN ('ui', 'api', 'seed', 'builtin')),
     metadata     TEXT NOT NULL DEFAULT '{}',
     created_by   TEXT,
     created_at   TEXT NOT NULL DEFAULT (strftime('%Y-%m-%dT%H:%M:%fZ', 'now')),
     updated_at   TEXT NOT NULL DEFAULT (strftime('%Y-%m-%dT%H:%M:%fZ', 'now'))
 );
 
-CREATE UNIQUE INDEX IF NOT EXISTS uq_hooks_global
-    ON agent_hooks (event, handler_type)
-    WHERE scope = 'global';
-
-CREATE UNIQUE INDEX IF NOT EXISTS uq_hooks_tenant
-    ON agent_hooks (tenant_id, event, handler_type)
-    WHERE scope = 'tenant';
-
-CREATE UNIQUE INDEX IF NOT EXISTS uq_hooks_agent
-    ON agent_hooks (tenant_id, agent_id, event, handler_type)
-    WHERE scope = 'agent';
-
 CREATE INDEX IF NOT EXISTS idx_hooks_lookup
     ON agent_hooks (tenant_id, agent_id, event)
     WHERE enabled = 1;