fix(framework): detect-platform get_gitea_token fail-loud on absent per-slot token (Patch 2b)

The per-agent identity resolution added to get_gitea_token() (Patch 2 /
#873) resolves an explicit identity (MOSAIC_GIT_IDENTITY env, or git
config mosaic.gitIdentity) and returns that identity's per-slot Gitea
token when present. Gap: when the identity resolves but its per-slot
token file is ABSENT for a recognized Gitea host, the function fell
through to the shared/default credential-loader token instead of
failing. API tooling (pr-create.sh, issue-create.sh, pr-review.sh) then
silently posted the PR/issue/review as the WRONG agent — e.g. a
reviewer seat's Gate-16 review getting attributed to the shared/default
identity — corrupting author≠reviewer separation while also masking
the missing-token misconfiguration. This surfaced most often on the
tea-stale API-fallback path, which is exactly when tooling leans on
get_gitea_token.

Fix: in the step-0 explicit-identity branch, when the per-slot token
for that identity is absent on a recognized Gitea host, print a stderr
diagnostic naming the identity, its source (env vs git config), the
host, and the expected token path, then return 1 instead of falling
through. All three callers already `return 1` on a nonzero
get_gitea_token, so fail-loud propagates with zero caller edits.

Scope (deliberate, minimal blast radius): explicit-identity-only. Plain
`git config user.name` is not an identity trigger — only
MOSAIC_GIT_IDENTITY / git config mosaic.gitIdentity count, so ordinary
shared/human repo usage is unaffected. Recognized-Gitea-hosts-only:
unrecognized hosts have no per-slot token scheme, so identity-set +
unknown-host still falls through unchanged (no fail-loud). The
MOSAIC_STRICT_IDENTITY opt-in discussed as a possible future extension
(gating the full `... > git username` chain) is deliberately NOT part
of this patch — deferred per spec as a later, separate proposal.

Red-first: stashed the source fix, ran the extended
test-gitea-token-identity.sh — 16 assertions failed exactly as
expected (shared token leaked, no fail-loud diagnostic). Restored the
fix — all green, full test:framework-shell chain passes. Backward
compat verified: the no-identity-requested case still returns the
shared token unchanged.

Extends test-gitea-token-identity.sh: the no-per-slot-token case (both
identity sources: env and git config) on a recognized host now asserts
nonzero return + empty stdout + stderr diagnostic naming
identity/source/host/expected path, instead of asserting a shared-token
fallback. Adds a same-identity cross-host case (token exists for one
host, absent for another → fail-loud on the host lacking it, no
cross-host token leak) and a scope-containment case (identity set +
unrecognized host → existing fall-through behavior unchanged, fail-loud
diagnostic does not fire).

Gates: shellcheck clean on both changed files, sanitization gate
(verify-sanitized.sh) passes, full test:framework-shell chain green.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0158NZqN2n2ymKFeJAZ4GUCb

packages/mosaic/framework/tools/git/detect-platform.sh

@@ -511,7 +511,11 @@ get_gitea_token() {
     #    (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="$(git config --get mosaic.gitIdentity 2>/dev/null || true)"
+    local _ident_src="MOSAIC_GIT_IDENTITY"
+    if [[ -z "$_ident" ]]; then
+        _ident="$(git config --get mosaic.gitIdentity 2>/dev/null || true)"
+        _ident_src="git config mosaic.gitIdentity"
+    fi
     if [[ -n "$_ident" ]]; then
         local _idpfx=""
         case "$host" in
@@ -524,6 +528,15 @@ get_gitea_token() {
                 cat "$_idtok"
                 return 0
             fi
+            # FAIL LOUD: an explicit git identity was requested for a recognized Gitea host,
+            # but no per-slot token exists for THAT identity. Refuse to fall through to the
+            # shared/default credential loader below — silently borrowing another slot's token
+            # would post PRs/issues/reviews under the WRONG agent (e.g. rev2's review attributed
+            # to coder3), corrupting Gate-16 author≠reviewer separation. Hard-stop instead so the
+            # caller aborts loudly rather than acting as the wrong identity.
+            echo "Error: git identity '$_ident' requested (via $_ident_src) for host '$host', but no per-slot token at $_idtok." >&2
+            echo "       Refusing to borrow another slot's token. Provision the per-slot token, or unset the identity to use shared credentials." >&2
+            return 1
         fi
     fi