docs(#14): remove broken links to fullsend-ai/fullsend guides

[fullsend-ai-coder]

Remove 8 broken documentation links (2 per file) from docs/code.md, docs/fix.md, docs/retro.md, and docs/review.md. These links pointed to customizing-with-agents-md.md and customizing-with-skills.md in the fullsend-ai/fullsend repo, which are inaccessible. The referenced guides have no local equivalent in this repo, so the link paragraphs are removed entirely. The surrounding section structure (headers, variables tables) is preserved.

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Closes #14

Post-script verification

• Branch is not main/master (agent/14-fix-broken-doc-links)
• Secret scan passed (gitleaks — bc7beaa9a57aed031c6ef1d9755c1f2cfca516da..HEAD)
• Pre-commit hooks passed (authoritative run on runner)
• Tests ran inside sandbox

[ralphbean]

/fs-fix make these links to the right doc in https://fullsend.sh instead

[fullsend-ai-coder]

🤖 Finished Fix · ✅ Success · Started 6:19 PM UTC · Completed 6:23 PM UTC
Commit: 0a95cac · View workflow run →

[fullsend-ai-retro]

🤖 Finished Retro · ✅ Success · Started 3:12 PM UTC · Completed 3:22 PM UTC
Commit: 7be3fdc · View workflow run →

[fullsend-ai-retro]

PR #15 fixed broken doc links across 4 files. The code agent chose link removal (the least preferred option) instead of replacement, requiring one rework cycle via /fs-fix. The fix agent then correctly applied replacement URLs in ~4 minutes. No review agent review was posted on this bot-authored PR, and 6 days elapsed before human approval of a trivial docs-only change. Two proposals filed: (1) guide the code agent to attempt issue-proposed fix options in preference order, and (2) investigate why the review agent consistently does not review bot-authored PRs in this repo.

Proposals filed

• Code agent should attempt issue-proposed fix options in preference order before falling back to removal (in fullsend-ai/agents)
• Review agent does not review coder-bot-authored PRs in the agents repo, creating a coverage gap (in fullsend-ai/fullsend)