fix(mcp): carry the index-drift guardrail into the per-project instructions

[AllanStepps]

The gap

SERVER_INSTRUCTIONS_NO_ROOT_INDEX omits the index-drift anti-pattern:

A file flagged "⚠ changed on disk after the last index sync" drifted from its index (most common on projects queried via projectPath, which have no live watcher).

That parenthetical names projectPath-reached projects as the audience most exposed to drift. A server whose root has no index serves nothing but projectPath queries — the variant says so itself ("pass its path as projectPath … for as many projects as you like in one session"). So the guidance is absent from the only audience that always meets the condition it describes.

An agent that receives the ⚠ changed on disk flag without ever having been told what it means can read a flagged response as a normal one. That makes this a correctness gap rather than an efficiency one: the flag exists precisely because codegraph declined to serve a possibly-mis-sliced body, and the agent is supposed to Read that file instead.

This deliberately does not re-send the playbook

__tests__/mcp-unindexed.test.ts pins that the per-project variant is not the full playbook, and the comment gives the reason ("keeps the common case tight"). I agree with that decision, so this PR does not touch it:

• expect(instructions).not.toMatch(/## How to query/) still passes, unchanged.
• The variant grows by 581 characters, not by the ~5.5 KB a full playbook would add.
• SERVER_INSTRUCTIONS is byte-identical before and after — verified by extracting both constants pre- and post-change and comparing.

Only the one bullet whose own text points at this audience moves, into a shared INDEX_DRIFT_GUARDRAIL constant that both variants interpolate, so a later wording edit cannot reach one variant and miss the other.

Tests

Three assertions added to the existing mcp-unindexed suite: both variants carry the guardrail, and the per-project one still is not the full playbook.

npx vitest run __tests__/mcp-unindexed.test.ts
  Test Files  1 passed (1)
       Tests  11 passed (11)

npm run build (tsc included) passes. __tests__/installer-targets.test.ts shows 2 failures on my machine under the CLAUDE_CONFIG_DIR describe — I verified those are pre-existing: identical on a clean checkout with my changes stashed. Node 22.23.2 (the suite needs node:sqlite, so ≥22.5 as documented; on Node 20 the DB-backed tests fail before reaching any of this).

How I ran into it

A global codegraph install, then work across several repos — so the server starts outside any index and every project is reached by projectPath. src/mcp/tools.ts:1643 already records a prior case where this variant's prose was not enough on its own and "the reporter had to add an AGENTS.md note"; I had written the same kind of note before tracking it to this constant.

Happy to narrow or reword if you would rather keep the variant leaner still.