Two changes to support pushing `/docs/ai/tools` into the top 10 for
"agent tools" (8,900 monthly volume, currently position 11).

**`docs/src/ai/tools.md`** — adds a concrete usage example to 7 tools:
`diagnostics`, `grep`, `fetch`, `edit_file`, `terminal`, `web_search`,
and `subagent`. Each example shows a realistic scenario rather than
restating the description.

**`docs/src/ai/external-agents.md`** — adds a single sentence
cross-linking to `tools.md` after the supported-agents intro paragraph,
for users who land on that page looking for what the built-in agent can
do.