[codex] Support marketplace plugin manifest fallback

[charlesgong-openai]

Summary

Support marketplace plugins whose source directory does not include a discoverable plugin manifest. Marketplace entries now synthesize a fallback .codex-plugin/plugin.json from supported marketplace.json fields and use that metadata for local detail reads, install, and non-curated cache refresh.

The install path stages a temporary source copy before writing the fallback manifest, so local marketplace checkout contents are not mutated. Existing source manifests still take precedence when present.

Details

• Adds a marketplace-entry manifest fallback that emits the plugin manifest fields supported today: version, description, keywords, skills, mcpServers, apps, hooks, and interface.
• Migrates all currently supported interface metadata from either top-level marketplace entry fields or nested interface, including descriptions, developer/category metadata, capabilities, URLs, default prompts, brand color, icons, logos, and screenshots.
• Maps author.name to interface.developerName and marketplace category to the plugin interface category; unsupported marketplace-only keys such as agents are not written into the generated plugin.json.
• Accepts skills as either a string or array in marketplace entries, using the first valid path for the synthesized plugin manifest because the current plugin manifest schema supports a single skills root.
• Uses the synthesized manifest only when the marketplace entry contributed supported metadata, preserving the existing missing-manifest error for bare name/source entries.

Validation

• CARGO_TARGET_DIR=/tmp/codex-target just test -p codex-app-server plugin_read_returns_invalid_request_when_plugin_manifest_is_missing passed: 1/1.
• CARGO_TARGET_DIR=/tmp/codex-target just test -p codex-core-plugins passed: 252/252.
• CARGO_TARGET_DIR=/tmp/codex-target just fix -p codex-core-plugins completed; it reported an existing unrelated large_enum_variant warning in core-plugins/src/manifest.rs.
• just fmt passed.