fix(git-wrappers): remove the guessed-login credential path from pr-review and issue-comment

With no --login, both wrappers resolved a login by GUESSING it from the repo host
(get_gitea_login_for_host / get_gitea_login), then looked that name up in
~/.config/tea/config.yml. On git.mosaicstack.dev the guess resolves to
mosaicstack-mos-dt-0, a SHARED account. get_gitea_token_for_login returns the
matching token string with no authentication check anywhere in its body, so it
returns rc=0 for a dead credential and the `|| get_gitea_token` fallback never
fires. The identity-aware resolver was unreachable on this path.

Measured on git.mosaicstack.dev with MOSAIC_GIT_IDENTITY set:

  get_gitea_token_for_login <guessed>  rc=0, token authenticates HTTP 401
  get_gitea_token <host>               rc=0, token authenticates HTTP 200

Control: the same endpoint with no credential returns 401.

The dead token is what made this visible; it is not the defect. Had the shared
token been alive, every seat's reviews and comments would have been authored by
the shared account, making Gate-16 author-is-not-reviewer unenforceable across
the fleet. A caller that passes no --login is asking to act as ITSELF, and the
guess answered a question nobody asked.

This removes the guess and its tea lookup from the no---login path in both
wrappers. That path now resolves the acting identity's own credential via
get_gitea_token, which fails loud on a fleet host when no identity resolves.
That refusal is the correct outcome and is deliberately not fallen back from.

Unchanged on purpose:

- get_gitea_token_for_login keeps its behaviour and its 53 assert_token pins in
  test-gitea-login-resolution.sh. pr-edit.sh and the explicit --login branches
  still use it; --login remains the only way to reach the tea store.
- No in-function verification was added. These wrappers verify every write by
  id-plus-author read-back against the credential-derived login, so a revoked
  token fails at the write with no misattribution. A GET /user pre-check would
  also hard-fail a live token scoped write:repository without read:user, which
  returns 403 while being fully comment-capable.

Tests: 28 pass. test-issue-close-fail-closed.sh fails identically on pristine
upstream (byte-identical output) and touches issue-close.sh, which this does not
modify. The wrapper test harness inherits an ambient MOSAIC_GIT_IDENTITY into its
sandbox HOME; tests were run with it unset.

Adversarial review by fargo, who found that reordering alone is a no-op wherever
MOSAIC_GIT_IDENTITY is unset, and that a 401/403 fail-closed pre-check would
reject correctly-scoped live tokens.

packages/mosaic/framework/tools/git/issue-comment.sh

@@ -102,9 +102,17 @@ gitea_resolve_api_for_login() {
             return 1
         }
     else
-        GITEA_API_TOKEN=$(get_gitea_token_for_login "$effective_login" "$host") \
-            || GITEA_API_TOKEN=$(get_gitea_token "$host") || {
-            echo "Error: Gitea token not found for login '$effective_login' (comment write/read-back)" >&2
+        # NO --login: the acting credential is this identity's own token and there
+        # is deliberately no tea-config fallback. get_gitea_token_for_login matches
+        # by login NAME and performs no authentication check, and with no --login
+        # that name was a HOST GUESS resolving to a shared account. A live shared
+        # token would therefore have authored every seat's comment as that
+        # account, making Gate-16 author-is-not-reviewer unenforceable fleet-wide;
+        # a dead one is only what made the defect visible. get_gitea_token fails
+        # loud on a fleet host when no identity resolves, and that refusal is the
+        # correct outcome, not a case to fall back from.
+        GITEA_API_TOKEN=$(get_gitea_token "$host") || {
+            echo "Error: no Gitea credential resolved for the acting identity on host '$host' (comment write/read-back). Set MOSAIC_GIT_IDENTITY=<agent-id>, or pass --login <name> to use a named tea credential." >&2
             return 1
         }
     fi
@@ -335,15 +343,12 @@ if [[ "$PLATFORM" == "github" ]]; then
     gh issue comment "$ISSUE_NUMBER" --body "$COMMENT"
     echo "Added comment to GitHub issue #$ISSUE_NUMBER"
 elif [[ "$PLATFORM" == "gitea" ]]; then
-    # Resolve the login this comment should be attributed to: the --login
-    # override when given, otherwise the detected default for this repo's host.
-    # A --login override always wins. Otherwise name this repo host's login only
-    # as a best effort: the login name merely selects a per-login token, and
-    # gitea_resolve_api_for_login falls back to the host credential
-    # (get_gitea_token) when no tea login is named, so the default credential
-    # still resolves even when the host tea has no matching login entry.
+    # A --login override selects a NAMED tea credential and is the only way to
+    # reach the tea store. With no --login there is deliberately no guess: the
+    # comment is attributed to this identity's own credential, resolved by
+    # gitea_resolve_api_for_login. The guess this replaced named a SHARED
+    # account, selecting an identity the caller never asked to act as.
     EFFECTIVE_LOGIN="$LOGIN_OVERRIDE"
-    [[ -n "$EFFECTIVE_LOGIN" ]] || EFFECTIVE_LOGIN=$(get_gitea_login 2>/dev/null || true)
 
     # Bind the REST endpoint + token to the effective login, then derive the
     # acting identity from that SAME credential (GET /user). The write below and