Add Codex marketplace manifest

Prompt: Jesse asked for a new worktree off the local superpowers dev branch to add the Codex manifest after diagnosing why github.com/obra/superpowers did not show installable Codex plugins.

Root cause: Codex marketplace sources expect a .agents/plugins/marketplace.json at the marketplace root. The superpowers repo only had the Claude marketplace file and the Codex plugin manifest, so Codex could configure the marketplace name but found no installable plugin entries.

Changes: add a repo-local Codex marketplace manifest for superpowers-dev that points at this same repository root via the same-root source pattern Codex already accepts; add a focused marketplace manifest test; remove the unsupported hooks field from .codex-plugin/plugin.json so the plugin validator accepts the manifest.

Validation: bash tests/codex/test-marketplace-manifest.sh; uv run --with PyYAML python /Users/jesse/.codex/skills/.system/plugin-creator/scripts/validate_plugin.py /Users/jesse/git/superpowers/superpowers/.worktrees/codex-marketplace-manifest; throwaway HOME codex plugin marketplace add/list/add; bash tests/codex-plugin-sync/test-sync-to-codex-plugin.sh; bash tests/kimi/test-plugin-manifest.sh; bash tests/shell-lint/test-lint-shell.sh; scripts/lint-shell.sh tests/codex/test-marketplace-manifest.sh.

.codex-plugin/plugin.json

@@ -21,7 +21,6 @@
     "workflow"
   ],
   "skills": "./skills/",
-  "hooks": "./hooks/hooks-codex.json",
   "interface": {
     "displayName": "Superpowers",
     "shortDescription": "Planning, TDD, debugging, and delivery workflows for coding agents",