feat(tools/git): per-agent Gitea identity (git-credential-mosaic + get_gitea_token)

Adds an opt-in per-agent Gitea identity so a fleet agent can push/commit/open
PRs under its own token instead of the single shared account, giving
cryptographic author-!=-reviewer separation (Gate-16).

Resolution priority (both tools): MOSAIC_GIT_IDENTITY env > git config
mosaic.gitIdentity (per-worktree, persists on disk) > git-supplied username
(git-credential-mosaic only). If the resolved identity has a token file at
~/.config/mosaic/secrets/gitea-tokens/gitea-{usc,mosaicstack}-<id>.token, that
identity is used; otherwise both tools fall through to the existing
shared-account path unchanged, so this is a no-op on any host without
per-slot tokens configured.

- tools/git/git-credential-mosaic: new git credential helper (get verb).
- tools/git/detect-platform.sh: get_gitea_token() gains the same identity
  resolution, prepended ahead of the existing shared-token logic, so API
  tooling (pr-create.sh, issue-create.sh, ...) authors under the same
  identity as git push/fetch.
- install.sh: explicit chmod +x for git-credential-mosaic (it ships without
  a .sh suffix, so the existing *.sh glob does not cover it); tools/** is
  already framework-owned so the file syncs automatically.
- tools/git/README.md: documents the feature, the one-time
  `git config credential.helper` registration step (deliberately not
  auto-wired — see README for why), and the PowerShell-parity decision
  (detect-platform.ps1 authenticates via tea logins, not a raw-token
  function, so there is nothing to port there).
- tools/git/test-git-credential-mosaic.sh,
  tools/git/test-gitea-token-identity.sh: new regression harnesses (red
  verified against the pre-patch code) covering identity-resolution
  priority, per-host token path selection, and shared-account fallback.
  Wired into package.json's test:framework-shell.

Upstreams Mos host-local tooling-patch kit (2026-07-23), Patches 1+2.

Closes #873

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

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 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"
+    "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 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"
   },
   "dependencies": {
     "@mosaicstack/brain": "workspace:*",