feat: add ty plugins remove to uninstall a plugin

[bborn]

What

Adds ty plugins remove <name> (aliases rm, uninstall) — the missing inverse of ty plugins add. Until now ty plugins could install and inspect plugins but the only way to uninstall one was to hand-delete a directory.

Behavior

$ ty plugins remove plan-code-review
Removed plugin "plan-code-review" (/Users/.../task/plugins/plugins/plan-code-review).
Note: it was part of a shared git checkout; its sibling plugins remain, and re-adding that source may restore it.

• Name matching: matches the manifest name shown by ty plugins list, falling back to the directory base name, so it works even when the two differ.
• Path safety: verifies the target lives strictly inside the plugins dir before deleting, so a corrupt/hostile manifest can't redirect the delete elsewhere.
• Collection-repo aware: a plugin nested inside a multi-plugin git checkout (e.g. the taskyou/plugins collection) has only its own subdirectory removed; the shared checkout and sibling plugins stay, and the user is warned that re-running add on the source may restore it.

Tests

cmd/task/plugins_remove_test.go covers:

• single-plugin repo removal (whole checkout gone, no longer discoverable)
• collection removal leaves siblings + reports inCheckout
• matching by directory name when it differs from the manifest name
• unknown-plugin errors instead of deleting anything

docs/plugins.md gets a Removing section.

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