fix(prompts): point tool-enablement guidance at Customize → Tools

[philmerrell]

Problem

The Customize epic deleted the composer settings drawer — docs/specs/customize-surface.md is explicit about it ("Does the settings icon survive? No."), and components/model-settings/ is gone. But the prompt and UI copy never followed, so we are still telling users to open "the gear icon next to the message input".

The composer today has attach (+), microphone, model dropdown and send. There is no gear. Tool enablement lives at /customize/tools, reached from the sidebar's Customize entry.

What changed

Model- and user-facing:

• system_prompt_builder.py — HANDLING MISSING TOOLS intro, step 2, and the worked example. The example is the worst of the set: it is a verbatim reply template, so the model reproduced the wrong instruction word-for-word.
• inference_api/chat/routes.py — the spreadsheet and PowerPoint attachment-diversion notes. These are appended to the user's own message, so the user reads the wrong direction in-thread, not just via the model.
• chat-input.component.ts — the "Enable Spreadsheet Analysis" nudge toast, found in the sweep.

Also drops "for this conversation" from the example reply: the toggles at /customize/tools are user-level preferences, not conversation-scoped.

Swept the rest of the tree for the same staleness: an admin placeholder claiming the prompt description is "shown to users in the conversation settings panel" (it is shown nowhere today), and five docstrings naming model-settings, the drawer, or "the My Skills page".

Not changed, but worth knowing

The code prompt is only the fallback. seed_bootstrap_data.py seeds no prompt text, so a dev/prod prompt edited through admin CRUD keeps whatever copy it was saved with. This PR does not fix a deployed environment — the DynamoDB prompt records need checking separately.

respectAgentLock: true is now a dead default. Every live caller passes false; the conversation surface that needed true was the drawer. The comment is corrected to stop naming a deleted surface, but the behavior is untouched — removing the parameter is a refactor, not a copy fix.

Cost note

This rewrites the system-prompt prefix once on deploy — an unavoidable one-time cache miss per session, not a recurring one. Nothing about the change is per-turn or order-dependent, so the prompt-cache contract is unaffected.

Verification

• Backend: 266 tests across agents/main_agent/core/ + test_stream_processor.py pass; 29 attachment/diversion tests pass.
• Frontend: full SPA suite, 3149/3149 pass.
• Final grep for gear icon / model settings panel / next to the message input / conversation settings panel / My Skills page across backend/src, frontend/ai.client/src, docs-site/src returns nothing.

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