fix(docs): remove stale plan/PLAN references

[N123999]

Related Issue

No linked issue. This is a docs-only cleanup for broken internal references; CONTRIBUTING allows opening this kind of PR directly.

Problem

packages/agent-core-v2/AGENTS.md and packages/agent-core-v2/docs/flag.md still pointed at plan/PLAN.md and GAP_ANALYSIS.md, which are not in the repository. Anyone following those links hits dead ends.

Git history shows these paths were never committed; the references date back to #1441 and #2451 already removed the same stale plan/PLAN.md link from flag.md but left the others.

Happy to adjust if these docs live elsewhere internally; I couldn't find them in git history.

What changed

• Updated the agent-core-v2 AGENTS.md intro blurb to point at existing documentation: docs/ (listed in the same file) and .agents/skills/agent-core-dev/.
• Removed the stale GAP_ANALYSIS.md reference from docs/flag.md.

No behavior or API changes.

Checklist

• I have read the CONTRIBUTING document.
• I have linked a related issue, or explained the problem above.
• I have added tests that prove my feature works. (N/A — docs-only)
• Ran gen-changesets skill, or this PR needs no changeset. (Not needed — docs-only)
• Ran gen-docs skill, or this PR needs no doc update. (This PR is the doc update)

[changeset-bot]

⚠️ No Changeset found

Latest commit: f2621d9

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