feat(plugins): tabulate plugin list output

[caseychow-oai]

Summary

• render codex plugin list as one table per marketplace with the marketplace manifest path shown above each table
• surface the installed plugin version in the CLI output by threading installed_version through marketplace listing state
• narrow the system-root exemption so only known bundled/runtime marketplaces skip missing-manifest failures, and keep VERSION empty for cached-but-unconfigured plugins

Rationale

The plugin list UX was hard to scan as a flat list and did not show which installed version was active. This change makes the CLI output easier to read in the real multi-marketplace case, keeps the plugin path visible, fixes the Sapphire regression where bundled/runtime marketplace roots were blocking plugin list, and addresses the two review findings that came out of the follow-up deep review.

Key Decisions

• kept the CLI output grouped per marketplace instead of one global table so the marketplace path can live with the rows it owns
• kept VERSION as the installed version, which means it is empty until a plugin is actually installed
• handled the bundled/runtime regression in the CLI snapshot validation path rather than widening app-server protocol or changing marketplace loading behavior
• narrowed the exemption to known system marketplace names plus expected system paths, so user-configured marketplaces under those directories still fail loudly
• gated installed_version on actual installed state so VERSION cannot show stale cache state for not installed rows

Validation

• just fmt
• Sapphire: cargo test -p codex-cli --test plugin_cli (14 passed; 0 failed)
• Sapphire smoke test: bundled/runtime roots still work
  • cargo run -q -p codex-cli -- plugin add sample@debug
  • cargo run -q -p codex-cli -- plugin list
  • verified the bundled/runtime-root scenario no longer errors and shows the expected marketplace table output
• Sapphire smoke test: custom marketplace under bundled path still errors
  • verified failed to load configured marketplace snapshot(s) for custom-marketplace
• Sapphire smoke test: cached-but-unconfigured plugin hides version
  • verified sample@debug not installed renders with an empty VERSION column

Sample Output

/tmp/custom-marketplace/plugin.json
NAME          VERSION  STATUS         DESCRIPTION
sample@debug  1.0.0    enabled        Debug sample plugin
other@local            not installed  Local development plugin