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

scripts/install.sh

@@ -30,10 +30,16 @@ done
 # Install git hooks into the current repo checkout
 HOOKS_SRC="$REPO_ROOT/scripts/git-hooks"
 if [[ -d "$HOOKS_SRC" ]]; then
+  # Unset inherited GIT_DIR/GIT_WORK_TREE — set by git when this script runs as
+  # a hook (e.g. pre-push), they override -C's directory-based discovery and
+  # would resolve against the invoking repo instead of $REPO_ROOT.
+  GIT_HOOKS_DIR="$(env -u GIT_DIR -u GIT_WORK_TREE git -C "$REPO_ROOT" rev-parse --git-path hooks)"
+  [[ "$GIT_HOOKS_DIR" = /* ]] || GIT_HOOKS_DIR="$REPO_ROOT/$GIT_HOOKS_DIR"
+  mkdir -p "$GIT_HOOKS_DIR"
   for hook_file in "$HOOKS_SRC"/*; do
     [[ -f "$hook_file" ]] || continue
     hook_name="$(basename "$hook_file")"
-    dest_hook="$REPO_ROOT/.git/hooks/$hook_name"
+    dest_hook="$GIT_HOOKS_DIR/$hook_name"
     cp "$hook_file" "$dest_hook"
     chmod +x "$dest_hook"
   done