docs(ai-agents): document skill auto-update for claude code

[RapidPoseidon]

What

Rewrites the "Keeping the skill up to date" section of docs/ai_agents.md to lead with the automatic option instead of only manual commands.

Claude Code refreshes marketplaces and updates their installed plugins in the background shortly after a session starts, but this defaults to off for every marketplace outside Anthropic's own — so nobody who installed our skill was getting it. The page now documents both ways to opt in:

• the one-time /plugin → Marketplaces → Enable auto-update toggle
• an extraKnownMarketplaces block with "autoUpdate": true for .claude/settings.json, so a team can commit it once

The manual commands stay, moved under their own heading. The skills CLI (Cursor / Copilot / Codex / Windsurf) has no auto-update mechanism, so npx skills update remains the only option there and the text now says so rather than implying parity.

Context

The skill drifting from the SDK is a real failure mode, not a hypothetical: the version pinned in the published skill is currently three minor versions behind PyPI, because the sync workflow in RapidataAI/skills has failed on every release since v3.16.7. This PR does not fix that — it only makes sure users receive updates once they are published again. The pipeline repair is tracked separately.

Verification

uv run --group docs mkdocs build passes; the new section renders on the AI Agents page. Remaining build warnings are pre-existing and unrelated.

🔗 Session: https://poseidon.rapidata.internal/chat/session-39aad07d