LOW: validate-config should warn on unresolved profile.yaml TODO placeholders

[PolyphonyRequiem]

Parent: #414
Severity: LOW — silent gap that lets a fresh bootstrap "validate green" while the profile is still empty

Symptom

A freshly-bootstrapped .polyphony-config/profile.yaml ships with placeholder TODOs:

# <!-- TODO: describe the project's domain in 1-2 sentences -->
domain: ""

# <!-- TODO: link to the architecture doc -->
architecture_url: ""

polyphony validate-config reports is_valid: true even when these placeholders are unfilled. Operators who skip the "edit profile.yaml" step (because validate-config reported clean) only discover the gap when the first agent run produces nonsensical output.

Fix

In ConfigValidator, when checking profile.yaml, scan for unresolved TODO markers (<!-- TODO: ... -->) and any string fields whose value matches the bootstrap-emitted placeholder pattern. Emit a V-warning (advisory, not blocking — bootstrap-emitted profile must still parse).

Format suggestion:

V0042 (advisory) profile.yaml contains 3 unresolved <!-- TODO: ... --> markers. Bootstrap placeholders should be resolved before first apex run.
  - line 12: domain
  - line 17: architecture_url
  - line 23: testing_strategy

Acceptance

• A fresh-bootstrap profile.yaml produces an advisory warning listing each unresolved TODO
• A fully-resolved profile.yaml produces no warnings
• Existing repos with profile.yaml files lacking TODO comments are unaffected

Cross-refs

• Twig-onboarding gap Feature/2582 pg 4 #7
• Architectural through-line: "verify the assumption explicitly" — bootstrap output should not pass validation while it still has placeholders