fix(framework): pr-review.sh -r/--repo + -H/--host overrides, UA header, repo preflight (Patch 5/5c)

Upstreams the pr-review.sh -r/--repo and -H/--host override family from the
host-local tooling-patches kit (patch 5 + 5c), adapted to the framework's
current REST-native review/comment implementation:

- -r/--repo <owner/repo>: explicit slug override, skipping git-remote slug
  inference, for reviewer worktrees whose origin is nonstandard or missing.
  Mirrors the established -r convention of the sibling wrappers (pr-view.sh,
  pr-diff.sh, pr-ci-wait.sh; #867). detect_platform is now tolerant of a
  missing/foreign git origin when -r is given (assumes gitea), matching the
  same sibling-wrapper convention.
- -H/--host <host>: explicit Gitea host override, skipping remote-host
  inference, so ambient CWD/remote state cannot cross-wire a review/comment to
  the wrong instance.
- Repo-exists preflight: when -r is used, GET .../repos/<slug> BEFORE any
  write. A wrong-host cross-wire now surfaces as a clear preflight error
  instead of an opaque write-404.
- `User-Agent: mosaic-pr-review` header added to every Gitea curl call (write,
  read-back, /user, PR-head read, preflight) — some Cloudflare-fronted Gitea
  hosts intermittently reject curl's default User-Agent.

Reproduce-first evidence (against origin/main baseline, fresh clone):
- -r/-H: `./pr-review.sh -n 1 -a comment -c x -r foo/bar` / `-H example.test`
  both failed with "Unknown option" pre-patch; both parse post-patch.
- User-Agent: `grep -c User-Agent pr-review.sh` was 0 pre-patch.
- New regression test (test-pr-review-repo-host-override.sh) fails against the
  baseline file and passes against the patched file (red-first confirmed).

Scope note — two sub-changes from the source kit's patch family (5b: an
issue_url-empty html_url readback fallback; 5d: REST-native reviews replacing
`tea approve/reject`) were investigated and found ALREADY SUPERSEDED on this
baseline by a more robust, already-tested implementation:
- 5d: the current gitea_submit_review_verified() already POSTs to
  /pulls/{n}/reviews (no `tea pr approve/reject` call exists in this file —
  `grep -c 'tea pr' pr-review.sh` is 0), with additional hardening (identity
  binding, PR-head commit pinning, current-head TOCTOU close-out) beyond what
  the source patch describes.
- 5b: comment verification already keys off `pull_request_url` (not
  `issue_url`) with full origin+path pinning (see gitea_create_comment_verified
  / test-pr-review-gitea-comment.sh cases 15b, 17), which already handles the
  Gitea empty-issue_url-on-PR-comments behavior the source patch targets, via a
  stricter mechanism than a bare html_url fallback would provide. Porting the
  source kit's simpler fallback verbatim would have been a regression.

Per HALT-on-no-repro discipline, only the two sub-changes that actually
reproduced against baseline (5, 5c) are included here.

Part of #891

packages/mosaic/package.json

@@ -25,7 +25,7 @@
     "lint": "eslint src",
     "typecheck": "tsc --noEmit",
     "test": "vitest run --passWithNoTests && pnpm run test:framework-shell",
-    "test:framework-shell": "python3 src/lease-broker/daemon_deadline_unittest.py && python3 src/lease-broker/normative_fragments_unittest.py && python3 src/lease-broker/receipt_challenge_unittest.py && python3 src/lease-broker/context_recovery_unittest.py && python3 src/lease-broker/recovery_runtime_unittest.py && python3 src/lease-broker/recovery_b1_adversarial_unittest.py && python3 src/lease-broker/framework_skill_portability_unittest.py && python3 src/mutator-gate/runtime_tools_unittest.py && python3 src/mutator-gate/runtime_launch_guard_unittest.py && python3 src/mutator-gate/version_coupling_unittest.py && python3 framework/tools/lease-broker/check-runtime-launches.py --root ../.. && bash framework/tools/codex/test-pr-diff-context.sh && bash framework/tools/qa/test-deps-preflight.sh && bash framework/tools/git/test-pr-review-gitea-comment.sh && bash framework/tools/git/test-ci-queue-wait-branch-absent.sh && bash framework/tools/git/test-git-credential-mosaic.sh && bash framework/tools/git/test-gitea-token-identity.sh && bash framework/tools/_scripts/test-install-ordering-guard.sh"
+    "test:framework-shell": "python3 src/lease-broker/daemon_deadline_unittest.py && python3 src/lease-broker/normative_fragments_unittest.py && python3 src/lease-broker/receipt_challenge_unittest.py && python3 src/lease-broker/context_recovery_unittest.py && python3 src/lease-broker/recovery_runtime_unittest.py && python3 src/lease-broker/recovery_b1_adversarial_unittest.py && python3 src/lease-broker/framework_skill_portability_unittest.py && python3 src/mutator-gate/runtime_tools_unittest.py && python3 src/mutator-gate/runtime_launch_guard_unittest.py && python3 src/mutator-gate/version_coupling_unittest.py && python3 framework/tools/lease-broker/check-runtime-launches.py --root ../.. && bash framework/tools/codex/test-pr-diff-context.sh && bash framework/tools/qa/test-deps-preflight.sh && bash framework/tools/git/test-pr-review-gitea-comment.sh && bash framework/tools/git/test-pr-review-repo-host-override.sh && bash framework/tools/git/test-ci-queue-wait-branch-absent.sh && bash framework/tools/git/test-git-credential-mosaic.sh && bash framework/tools/git/test-gitea-token-identity.sh && bash framework/tools/_scripts/test-install-ordering-guard.sh"
   },
   "dependencies": {
     "@mosaicstack/brain": "workspace:*",