Follow-up to #2336 (umbrella) / PR #2463 (partial sweep). Covers Area 6 deferred from that PR.

Scope

• `docs/tools/slash-commands.md` — remove `/think` entries (lines 17, 104, 122 in gut(auto-reply): finish thinking-level and model-selection sweep — follow-up to #2335 #2336 body).
• `docs/concepts/context.md:139` — remove `/think` from directive list.
• `docs/web/tui.md:94` — remove `/think` TUI documentation.
• `docs/channels/group-messages.md:18,70` — remove `/think high` examples (suggest `/verbose on` instead).

AC

• `git grep "/think" docs/` returns zero hits.
• Docs link check passes: `pnpm docs:check-links`.

Context

• Parent: gut(auto-reply): finish thinking-level and model-selection sweep — follow-up to #2335 #2336
• Sibling shipped: PR gut(auto-reply): partial thinking-level + model-selection sweep — Areas 1+2+4 of #2336 #2463