Expose resource diagnostics in the TUI

[tylergibbs1]

Summary

• add a built-in /diagnostics command that shows every current resource diagnostic
• render severity, resource kind, optional name, optional source path, and remediation message in the existing scrollable TUI command modal and in print mode
• reserve diagnostics.md so prompt expansion cannot shadow the command
• document the command and cover empty, populated, dismissal, autocomplete, print-mode, and post-reload behavior

Why

Tau already retains non-fatal resource diagnostics, but users could only see their count through /session or /reload. The detailed messages and source paths were not reachable from a registered user-facing command, making ignored or invalid resources difficult to repair.

Validation

• env -u OPENAI_API_KEY uv run pytest -q — 1,155 passed
• uv run ruff format --check .
• uv run ruff check .
• uv run mypy
• git diff --check

Closes #459