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

plugins/gitea/plugin.json

@@ -1,5 +1,4 @@
 {
-  "agents": "agents/",
   "author": {
     "email": "defame1297@rkdr.net",
     "name": "Defame1297"
@@ -16,8 +15,5 @@
   "license": "MIT",
   "mcpServers": ".mcp.json",
   "name": "gitea",
-  "skills": [
-    "skills/"
-  ],
   "version": "1.0.0"
 }