chore: fold maintainer answers into the threat model

@mgubaidullin answered waves 1-3 on #1642. Every hypothesis in the first
draft is now confirmed, corrected, or superseded: 57 documented /
40 maintainer / 0 inferred, down from 28 inferred.

Rulings that changed the model rather than confirming it:

- Q11 corrected the draft. The non-invalidating
  /platform/internal/sources/{projectId}/{filename} route is by design,
  not an oversight; the single-use guarantee is scoped to the
  whole-project route only.
- Q5 confirmed the negative side-effect inventory is accurate but
  declined to commit to it as an invariant, so it is recorded as a
  description of the current release rather than a promise.
- Q1 ruled the default platform.password an install-time convenience
  set via secret or env var and changed through the UI, making reports
  against it OUT-OF-MODEL: non-default-build.

Q9 was answered "already fixed", which rebinds the document. CsrfFilter
(d211d71f), PathUtils name validation (5e425249) and the Kubernetes
resource allow-list all landed after the 4.22.0 tag, so the model now
binds to main at 677a293a and says explicitly that a report against
4.22.0 or earlier is triaged against a model without those three. CSRF
moved from a section 4.9 false friend to a claimed property in 4.8.

The Kubernetes resource validation is described as defence in depth and
explicitly not a boundary: it does not cover the builder pod path,
build.sh, or the Docker compose path.

Q13 and Q14, on document ownership and revision policy, remain open.
Appendix B records every ruling so a disputed disposition can be traced
to the answer behind it.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>