docs(#3861): fix documentation nav inconsistencies

[fullsend-ai-coder]

Three documentation inconsistencies from PR #3398 are resolved:

1. Add VitePress sidebar entry for default-vs-custom.md so the page
   is discoverable via site navigation.
2. Align link text with target page H1 headings — change
   "Configuring..." to "Customizing..." across agent docs and
   guides README.
3. Update sidebar title from "Building Custom Agents" to
   "Building custom agents from scratch" to match the page H1.

---

Closes #3861

Post-script verification

• Branch is not main/master (agent/3861-fix-docs-inconsistencies)
• Secret scan passed (gitleaks — b612dab8952bd07379cf21d78d16f4d3b7b41f16..HEAD)
• Pre-commit hooks passed (authoritative run on runner)
• Tests ran inside sandbox

[github-actions]

E2E tests did not run

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For other contributors, a maintainer must add the ok-to-test label after the latest push.

See E2E testing guide for details.

[github-actions]

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Commit: 9cd4c713af9714e883e37a35aa6cd31f8759c9bd

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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[fullsend-ai-retro]

🤖 Finished Retro · ✅ Success · Started 10:10 AM UTC · Completed 10:29 AM UTC
Commit: 9cd4c71 · View workflow run →

[fullsend-ai-retro]

Retro: PR #3868 — docs(#3861): fix documentation nav inconsistencies

Workflow quality: Good overall. The full retro→triage→code pipeline worked well for this concrete docs fix. The code agent correctly fixed all 3 identified inconsistencies from PR #3398 in a single clean commit with 8 precise file changes, completing in ~11 minutes.

Key finding: review agent never dispatched. No review agent ran on this PR — or on any of the 4 most recent code-agent PRs in this repo (#3868, #3448, #3286, #3156). The post-code.sh script applies the ready-for-review label as a workaround for bot users failing the pull_request_target.opened authorization check, but the label-based dispatch path is not triggering.

Proposals filed

1. Review agent dispatch gap for code-agent PRs — The label-based bot-to-bot handoff described in ADR 0054 is not working for PR labels. Code-agent PRs skip automated review entirely.
2. VitePress sidebar still missing entries for behaviour-testing docs — Two dev docs added in recent PRs are missing from the sidebar config, the same class of issue PR docs(#3861): fix documentation nav inconsistencies #3868 fixed.

Proposals filed

• Review agent never dispatched for code-agent PRs due to broken label-based handoff (in fullsend-ai/fullsend)
• VitePress sidebar missing entries for behaviour-testing and behaviour-drivers docs (in fullsend-ai/fullsend)