docs(mcp): retire stale addon setup guidance

[huangzesen]

Root cause

The active MCP manual described curated-addon setup but did not state the current TUI command boundary. That gap let an agent recommend the retired /addon command and misdescribe /mcp as a guided setup screen during a WhatsApp setup request.

Current TUI behavior is deterministic: /addon was retired; /mcp is the only command for this surface, and it is read-only configuration/status inspection.

Fix

• state the retired /addon / read-only /mcp boundary in the MCP manual router
• keep curated setup on the existing provider-doc + exact-authorization path instead of redirecting users to a nonexistent setup UI
• add a semantic regression against the real shipped router/reference so stale command guidance cannot return silently

No TUI command, alias, wizard, runtime guard, provider schema, or WhatsApp credential behavior is added here.

Validation

Failing-first: the new regression failed against the unchanged base because the shipped manual had no current-command/read-only contract.

Final parent gates:

• python -m pytest -q tests/test_mcp_capability.py — 28 passed
• related MCP/manual/identity/docs tests — 117 passed
• python -m pytest -q tests/test_architecture_documents.py — 3 passed
• python scripts/check_docs_governance.py --check — 277 documents validated
• python -m py_compile tests/test_mcp_capability.py
• git diff --check

Requested from Jason's screenshot showing the stale /addon recommendation. Personal-account WhatsApp transport is a separate product feature; this PR corrects the current shipped setup contract without pretending that such transport already exists.