docs(prompt): require conventional commit-style PR titles

[tcballard]

Summary

Makes the PR-title convention explicit in the PR-authoring guidance. The repository
squash-merges pull requests, so the PR title becomes the squashed commit's subject
on main — it must therefore follow the commit message standard already defined in
rac-agent-commit-guidelines.md. The guidance previously specified the body format
but said nothing about the title.

Adds to rac/prompts/rac-agent-pr-guidelines.md:

• An Instructions paragraph requiring the title in type(area): imperative summary
  form, explaining the squash-merge rationale.
• A Constraints bullet forbidding prose titles like "Implement X" / "Add feature Y".

ADR Trace

No ADR change. Aligns the PR-authoring prompt with the existing commit message
standard (rac-agent-commit-guidelines.md); both are agent operating-guidance
prompt artifacts under ADR-047.

Scope

Included

• Two additive edits to rac/prompts/rac-agent-pr-guidelines.md (Instructions
  rationale + Constraints rule).

Excluded

• No change to the commit message standard itself, which already defines the
  format; this only states that PR titles follow it.
• No code change.

Verification

Ran

• rac validate rac/prompts/rac-agent-pr-guidelines.md → PASS, 0 errors/warnings.
• rac validate rac/ → 314 valid, 0 invalid.
• rac relationships rac/ --validate → 0 issues.
• rac review rac/ → exit 0.
• rac export rac/ --agent-rules --check → in-sync (a prompt edit leaves the
  decisions block untouched).

Review Path

rac/prompts/rac-agent-pr-guidelines.md — the Instructions paragraph and the new
Constraints bullet.

Notes For Reviewer

This PR's own title follows the new rule, as does the companion ADR-086 PR (#207),
which was retitled feat(telemetry): add enterprise telemetry hard-lock.

Implementation Process

Authored with AI assistance under the decision-contract workflow; final review and
acceptance are the maintainer's.