fix(prompt-hook): desktop reminders and other sessions' messages no longer drive the hook (#2798)

Claude Code's desktop app puts <system-reminder> blocks (which local
sessions are linked, and similar status) in front of the prompt it
submits, and UserPromptSubmit hooks receive them inside `prompt`. The
hook gated, picked the project and explored on the whole string, so the
reminder's words drove it: a linked session titled after its work, like
"trace-order-flow", carries structural keywords, so every turn, a bare
"yes" included, got a full explore of the reminder. A reminder in front
of a task notification or a subagent hand-back also defeated the
start-anchored envelope checks, and a message from another session
(a <cross-session-message from=...> envelope) was never skipped at all.

The hook now removes every complete <system-reminder> block before the
envelope checks, the gate, project selection and explore, returns early
when nothing is left, and skips a prompt that is entirely a
<cross-session-message> envelope, as it already did for task
notifications and subagent hand-backs. The scan stops at the last
closing tag and the attribute run stops at `<`, so a pasted prompt full
of unclosed tags stays linear (520 KB in under 1 ms; the `[^>]*` form
took 83 ms on a tenth of that and grows quadratically).

Validation:
- Prompt shapes come from the Claude Code desktop transcripts on this
  machine (314 transcripts): reminders always lead, one to four of
  them, separated by one or two newlines, and cross-session messages
  follow one to four reminders.
- frontload-hook.test.ts: 15 new tests (8 spawn the built hook with
  Claude Code-shaped payloads) fail on main and pass with the fix; the
  file's 48 other tests pass on both.
- Built CLI, private HOME, 13 payloads: on main every prompt behind a
  reminder got a full explore of the reminder (2.1 to 5.0 KB); with the
  fix the 8 that are not user input inject nothing and exit in about
  200 ms instead of 680 ms, and a question behind reminders injects
  byte-for-byte what the same question alone does.

Issue #2635. Contributor PR #2642 (yang0228) was read for guidance.

Fixes #2635

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/bin/codegraph.ts

@@ -53,7 +53,7 @@ try {
 import { Command } from 'commander';
 import * as path from 'path';
 import * as fs from 'fs';
-import { getCodeGraphDir, isInitialized, hasSchemalessDb, hasForeignDbFile, unsafeIndexRootReason, findNearestCodeGraphRoot, planFrontload, isTaskNotification, isAgentMessage, hasStructuralKeyword, extractCodeTokens, capPromptHookInjection, codeGraphDirName, DEFAULT_CODEGRAPH_DIR } from '../directory';
+import { getCodeGraphDir, isInitialized, hasSchemalessDb, hasForeignDbFile, unsafeIndexRootReason, findNearestCodeGraphRoot, planFrontload, isTaskNotification, isAgentMessage, isCrossSessionMessage, stripSystemReminders, hasStructuralKeyword, extractCodeTokens, capPromptHookInjection, codeGraphDirName, DEFAULT_CODEGRAPH_DIR } from '../directory';
 import { extractProseCandidates } from '../search/identifier-segments';
 import { detectWorktreeIndexMismatch, worktreeMismatchWarning } from '../sync/worktree';
 import { createShimmerProgress } from '../ui/shimmer-progress';
@@ -1512,11 +1512,14 @@ program
 
       let input: { prompt?: string; cwd?: string } = {};
       try { input = JSON.parse(raw); } catch { return; }
-      const prompt = String(input.prompt || '');
-      // System-injected task notifications and subagent hand-backs are not
-      // user prompts: exit before any project lookup or explore work (#1832,
-      // #2184).
-      if (isTaskNotification(prompt) || isAgentMessage(prompt)) return;
+      // The desktop app's <system-reminder> blocks are host status, not what
+      // the user typed: gate, pick the project and explore on the rest
+      // (#2635).
+      const prompt = stripSystemReminders(String(input.prompt || ''));
+      // System-injected task notifications, subagent hand-backs and messages
+      // from other sessions are not user prompts, and neither is nothing:
+      // exit before any project lookup or explore work (#1832, #2184, #2635).
+      if (!prompt || isTaskNotification(prompt) || isAgentMessage(prompt) || isCrossSessionMessage(prompt)) return;
 
       // Gate telemetry: how often each tier fires vs. no-ops — counter names
       // only, NEVER prompt content (see TELEMETRY.md). This is the data that