feat: implement /loop command with fixed and dynamic scheduling (#621)

* feat: implement /loop command with fixed and dynamic scheduling modes

Enable cron tools and /loop skill without the AGENT_TRIGGERS build flag
by removing feature guards from tools.ts, REPL.tsx, and skill registration.
The isKairosCronEnabled() runtime gate now enables cron unconditionally for
open builds while preserving the GrowthBook kill switch for ant builds.

The /loop skill supports four modes: fixed-interval with prompt, fixed-interval
maintenance, dynamic-prompt (self-pacing), and dynamic maintenance (bare /loop).

* chore: remove unused DEFAULT_INTERVAL constant from loop skill

* revert: drop infra changes, scope PR to /loop skill rewrite only

The cron activation layer (AGENT_TRIGGERS guard removal, isKairosCronEnabled
hardcode) is covered by an in-flight stack (#633, #639). Scope this PR to
just the loop.ts rewrite and its tests so it can land cleanly on top.

* fix: restore infra changes needed for /loop in open build

Bun's constant folder evaluates feature('AGENT_TRIGGERS') at bundle time
through the bun:bundle shim — even when the flag is flipped to true in
build.ts, the folded value is cached from the previous build and stays false.
This means the feature-gated require() blocks for cron tools, useScheduledTasks,
and loop skill registration all compile to dead code regardless of the flag.

Fix by removing the AGENT_TRIGGERS guards from the specific paths /loop needs:
- tools.ts: cron tools always registered (isEnabled gates visibility)
- REPL.tsx: useScheduledTasks always mounted
- index.ts: registerLoopSkill via static import, called unconditionally
- prompt.ts: isKairosCronEnabled() bypasses feature flag for non-ant builds

* fix: replace backslash line continuations with explicit delimiters in loop prompts

The backslash-newline sequences inside template literals were acting as
line continuations, collapsing newlines and merging prompt content with
surrounding instruction text. Replace with --- BEGIN/END --- markers
for unambiguous delimiting.

Also add tests for trailing "every" clause parsing, human-readable unit
normalization, and the non-interval "check every PR" case.

* fix: remove remaining AGENT_TRIGGERS guards from print.ts and constants/tools.ts

Completes the cron guard removal started in the previous commit.
The cron scheduler in non-interactive (-p) mode was dead because
print.ts still gated cronSchedulerModule/cronGate requires behind
feature('AGENT_TRIGGERS'), which Bun constant-folds to false in open
builds. Similarly, cron tool names were absent from
IN_PROCESS_TEAMMATE_ALLOWED_TOOLS.

Remove all three guards so the scheduler initialises (gated at runtime
by isKairosCronEnabled) and cron tools are allowed for in-process
teammates in all builds.

src/screens/REPL.tsx

@@ -196,7 +196,7 @@ const PROACTIVE_NO_OP_SUBSCRIBE = (_cb: () => void) => () => { };
 const PROACTIVE_FALSE = () => false;
 const SUGGEST_BG_PR_NOOP = (_p: string, _n: string): boolean => false;
 const useProactive = feature('PROACTIVE') || feature('KAIROS') ? require('../proactive/useProactive.js').useProactive : null;
-const useScheduledTasks = feature('AGENT_TRIGGERS') ? require('../hooks/useScheduledTasks.js').useScheduledTasks : null;
+const useScheduledTasks = require('../hooks/useScheduledTasks.js').useScheduledTasks;
 /* eslint-enable @typescript-eslint/no-require-imports */
 import { isAgentSwarmsEnabled } from '../utils/agentSwarmsEnabled.js';
 import { useTaskListWatcher } from '../hooks/useTaskListWatcher.js';
@@ -4076,21 +4076,13 @@ export function REPL({
   });
 
   // Scheduled tasks from .claude/scheduled_tasks.json (CronCreate/Delete/List)
-  if (feature('AGENT_TRIGGERS')) {
-    // Assistant mode bypasses the isLoading gate (the proactive tick →
-    // Sleep → tick loop would otherwise starve the scheduler).
-    // kairosEnabled is set once in initialState (main.tsx) and never mutated — no
-    // subscription needed. The tengu_kairos_cron runtime gate is checked inside
-    // useScheduledTasks's effect (not here) since wrapping a hook call in a dynamic
-    // condition would break rules-of-hooks.
-    const assistantMode = store.getState().kairosEnabled;
-    // biome-ignore lint/correctness/useHookAtTopLevel: feature() is a compile-time constant
-    useScheduledTasks!({
-      isLoading,
-      assistantMode,
-      setMessages
-    });
-  }
+  // and session-only /loop runs.
+  const assistantMode = store.getState().kairosEnabled;
+  useScheduledTasks({
+    isLoading,
+    assistantMode,
+    setMessages
+  });
 
   // Note: Permission polling is now handled by useInboxPoller
   // - Workers receive permission responses via mailbox messages