feat: uninstall installed plugins (#37) - #228

What

Closes #37. Adds !plugin uninstall <name>, the inverse of !plugin install (#36): it removes the plugin from the persistent manifest and deletes its staged copy, so it no longer auto-loads at start-up. !plugin list shows the remaining set.

!plugin uninstall analytics   # drop from the auto-load set + delete staged copy

It is orthogonal to !plugin unload: uninstall changes only the persistent set — a plugin already loaded this session stays active until you !plugin unload it or restart. (Decided with the issue author.)

How

• src/plugin/engine.rs — PluginEngineCommand::UninstallPlugin. The handler normalizes the name (foo / foo.lua both work via get_plugin_name), errors if the plugin isn't installed, updates the manifest, then deletes the staged <name>.lua. A missing file is tolerated; other IO errors warn but don't abort (the manifest, which drives auto-load, is already updated).
• Stdlib-deletion guard — the deletion never targets a bundled STDLIB file (scope.lua / shell.lua). !plugin install rejects reserved names, so a manifest listing one can only come from a hand-edited file; silently losing a user-customized scope.lua to it would be a nasty surprise, so the file is preserved (the bogus manifest entry is still cleaned). This closed the one defense-in-depth gap flagged by an adversarial review of the diff.
• src/plugin/installed.rs — Installed::remove (returns whether it was present).
• src/inputs/inputs_task.rs — the !plugin handler gains the uninstall subcommand (arg-required), alongside the existing load/reload/install/list/unload.
• Docs: README command table + Plugins prose, CLAUDE.md, plugins/README.md.

Tests

• installed.rs unit test for remove (presence reporting + dedup-safety).
• tests/tui_e2e.rs:
  • uninstall removes the manifest entry and deletes the staged file;
  • uninstalling something not installed is a clear error;
  • the stdlib-safety guard: a corrupt manifest listing scope has the entry cleaned but scope.lua survives.

All 232 unit + 23 e2e tests pass (1 e2e ignored on macOS per the existing PTY-baud limitation); tree is warning-clean under #![deny(warnings)].

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