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/framework/install.sh

@@ -639,6 +639,10 @@ reconcile_framework_files
 # Ensure tool scripts are executable
 find "$TARGET_DIR/tools" -name "*.sh" -exec chmod +x {} + 2>/dev/null || true
 find "$TARGET_DIR/tools/_scripts" -type f -exec chmod +x {} + 2>/dev/null || true
+# git-credential-mosaic (per-agent Gitea identity helper) ships without a .sh
+# suffix — git resolves credential helpers by exact name/path, not extension —
+# so the *.sh glob above does not cover it; chmod it explicitly.
+[[ -f "$TARGET_DIR/tools/git/git-credential-mosaic" ]] && chmod +x "$TARGET_DIR/tools/git/git-credential-mosaic" 2>/dev/null || true
 
 ok "Framework synced to $TARGET_DIR"