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/tools/git/detect-platform.sh

@@ -505,6 +505,28 @@ get_gitea_token() {
     script_dir="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
     local cred_loader="$script_dir/../_lib/credentials.sh"
 
+    # 0. Per-agent identity (Gate-16 author≠reviewer). If MOSAIC_GIT_IDENTITY, or the
+    #    per-worktree `git config mosaic.gitIdentity`, resolves to an agent that has a
+    #    stored per-slot token for this host, act AS that agent so API tooling
+    #    (pr-create, issue-create, …) authors under the right identity — matching the
+    #    git credential helper. Backward-compatible: nothing resolvable → shared logic below.
+    local _ident="${MOSAIC_GIT_IDENTITY:-}"
+    [[ -z "$_ident" ]] && _ident="$(git config --get mosaic.gitIdentity 2>/dev/null || true)"
+    if [[ -n "$_ident" ]]; then
+        local _idpfx=""
+        case "$host" in
+            git.uscllc.com)      _idpfx=gitea-usc ;;
+            git.mosaicstack.dev) _idpfx=gitea-mosaicstack ;;
+        esac
+        if [[ -n "$_idpfx" ]]; then
+            local _idtok="$HOME/.config/mosaic/secrets/gitea-tokens/${_idpfx}-${_ident}.token"
+            if [[ -r "$_idtok" ]]; then
+                cat "$_idtok"
+                return 0
+            fi
+        fi
+    fi
+
     # 1. Mosaic credential loader (host → service mapping, run in subshell to avoid polluting env)
     if [[ -f "$cred_loader" ]]; then
         local token