feat: per-agent Gitea identity resolution (#873)

Closes #873

Mos (id-11) Gate-16 merge: independent APPROVE @4b472a22 (author-blocker dissolved via (a) re-author, identical tree hash to tech-approved head), author id2 != approver id11, clean mosaic-coder commit-author, CI green wp1985. Framework train COMPLETE 6/6.

Co-authored-by: jason.woltje <jason@diversecanvas.com>
Co-committed-by: jason.woltje <jason@diversecanvas.com>

packages/mosaic/framework/tools/git/git-credential-mosaic

@@ -0,0 +1,69 @@
+#!/bin/bash
+# git-credential-mosaic — git credential helper — resolves Gitea tokens from
+# the Mosaic credential store at runtime so remote URLs never embed secrets.
+#
+# Install (one-time, per clone or globally):
+#   git config credential.helper "$HOME/.config/mosaic/tools/git/git-credential-mosaic"
+#   # or, fleet-wide: git config --global credential.helper "$HOME/.config/mosaic/tools/git/git-credential-mosaic"
+#
+# Per-agent Gate-16 identity (author != reviewer separation):
+#   git config mosaic.gitIdentity <agent-id>   # per-worktree, persists on disk
+#   # or: export MOSAIC_GIT_IDENTITY=<agent-id>
+#
+# Resolution priority: MOSAIC_GIT_IDENTITY env > git config mosaic.gitIdentity
+# (per-worktree, survives across non-persistent shells) > git-supplied username
+# (credential.username / URL). When the resolved identity has a matching
+# per-agent token file, use it instead of the shared account. Backward
+# compatible: nothing resolvable -> shared token (unchanged behavior).
+[ "$1" = "get" ] || exit 0
+host=""; username_in=""
+while IFS= read -r line; do
+  [ -z "$line" ] && break
+  case "$line" in
+    host=*)     host=${line#host=};;
+    username=*) username_in=${line#username=};;
+  esac
+done
+# Per-agent identity resolution (Gate-16 author≠reviewer separation).
+# Priority: MOSAIC_GIT_IDENTITY env  >  git config mosaic.gitIdentity (per-worktree,
+# survives across non-persistent shells)  >  git-supplied username (credential.username
+# / URL). When the resolved identity has a matching per-agent token, use it instead of
+# the shared account. Backward-compatible: nothing resolvable → shared token.
+ident="$MOSAIC_GIT_IDENTITY"
+[ -z "$ident" ] && ident=$(git config --get mosaic.gitIdentity 2>/dev/null)
+[ -z "$ident" ] && ident="$username_in"
+if [ -n "$ident" ]; then
+  case "$host" in
+    git.uscllc.com)      idpfx=gitea-usc;;
+    git.mosaicstack.dev) idpfx=gitea-mosaicstack;;
+    *) idpfx="";;
+  esac
+  if [ -n "$idpfx" ]; then
+    idtok="$HOME/.config/mosaic/secrets/gitea-tokens/${idpfx}-${ident}.token"
+    if [ -r "$idtok" ]; then
+      echo "username=${ident}"
+      echo "password=$(cat "$idtok")"
+      exit 0
+    fi
+  fi
+fi
+case "$host" in
+  git.uscllc.com)      svc=gitea-usc;;
+  git.mosaicstack.dev) svc=gitea-mosaicstack;;
+  *) exit 0;;
+esac
+# Script-relative (not $HOME-absolute) so this resolves correctly regardless
+# of where the framework installer places tools/ under $HOME — mirrors
+# detect-platform.sh's own cred_loader resolution in this same directory.
+script_dir="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+# shellcheck source=../_lib/credentials.sh
+source "$script_dir/../_lib/credentials.sh"
+load_credentials "$svc" >/dev/null 2>&1 || exit 0
+# GITEA_USER is not populated by load_credentials (it only exports
+# GITEA_URL/GITEA_TOKEN for gitea-*), so this fallback is normally taken. Gitea's
+# git-over-HTTP auth authenticates from the token itself (the password field),
+# not from the username string, so any non-empty placeholder works here — this
+# is deliberately NOT a real account name (framework files must stay
+# operator-agnostic; see tools/quality/scripts/verify-sanitized.sh).
+echo "username=${GITEA_USER:-git}"
+echo "password=$GITEA_TOKEN"