fix(install): resolve git hooks dir via git plumbing, drop stale manifest fields

scripts/install.sh hardcoded $REPO_ROOT/.git/hooks, which breaks under any
git worktree checkout (.git is a file there, not a directory) — this is
what blocks every worktree-based agent from pushing cleanly. Resolve the
hooks directory via `git rev-parse --git-path hooks` instead, normalizing
to an absolute path since git returns it relative to the queried repo root
for plain checkouts but absolute for worktrees.

Also drops `agents`/`skills` fields from plugins/bin, plugins/core, and
plugins/gitea plugin.json where the referenced directories don't exist on
main yet (bin never had an agents/ dir; core and gitea's real skill/agent
content is still pending merge from an in-flight branch) — these were
failing scripts/check-manifests.sh and blocking pushes for unrelated work.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FNJWdVvdgvZCHi1hZGqgVQ

tests/test-git-hooks-install.sh

@@ -22,7 +22,7 @@ TEMP_HOME="$(mktemp -d)"
 trap 'rm -rf "$TEMP_REPO" "$TEMP_HOME"' EXIT
 
 mkdir -p "$TEMP_REPO/scripts"
-mkdir -p "$TEMP_REPO/.git/hooks"
+git -C "$TEMP_REPO" init -q
 
 # Minimal deploy-manifest.sh — empty deploy lists so install.sh reaches the
 # hook-copy block without attempting to copy files that don't exist in the
@@ -90,6 +90,28 @@ else
 fi
 rmdir "$HOOKS_SRC/not-a-hook"
 
+# ---------------------------------------------------------------------------
+echo ""
+echo "--- hook install: resolves correctly when GIT_DIR is inherited (hook context) ---"
+# ---------------------------------------------------------------------------
+
+# Git sets GIT_DIR/GIT_WORK_TREE for child processes when this script itself
+# runs as a hook (e.g. pre-push). Simulate that and confirm install.sh still
+# resolves hooks against $TEMP_REPO, not the inherited GIT_DIR.
+OTHER_REPO="$(mktemp -d)"
+git -C "$OTHER_REPO" init -q
+if HOME="$TEMP_HOME" GIT_DIR="$OTHER_REPO/.git" GIT_WORK_TREE="$OTHER_REPO" \
+    bash "$TEMP_REPO/scripts/install.sh" > /dev/null 2>&1; then
+  if [[ -f "$TEMP_REPO/.git/hooks/post-push" ]]; then
+    pass "resolves \$TEMP_REPO/.git/hooks/ even with inherited GIT_DIR"
+  else
+    fail "installed into inherited GIT_DIR instead of \$TEMP_REPO"
+  fi
+else
+  fail "install.sh failed when GIT_DIR/GIT_WORK_TREE were inherited"
+fi
+rm -rf "$OTHER_REPO"
+
 # ---------------------------------------------------------------------------
 echo ""
 echo "--- hook install: idempotent ---"