Prune stale cache copies when installing

[diegohb]

What to build

Running the suite's CLI install (either scope, including a no-op) must also clear this package's non-pinned copies from OpenCode's package cache, so OpenCode re-fetches the just-installed version on next start instead of reusing a stale or partial cached extraction indefinitely.

The cache location mirrors OpenCode's own resolution: join(XDG_CACHE_HOME || join(homedir(), ".cache"), "opencode", "packages"). This holds across platforms (it is XDG-shaped even on macOS/Windows; only homedir() is OS-specific).

On install:

• remove opencode-architect, opencode-architect@latest, and opencode-architect@<running version>
• keep other pinned versions (opencode-architect@0.6.0, etc.) and every other package's cache dir
• perform this on every install invocation, including when the install is a no-op
• treat deletion as best-effort: a removal failure warns but does not fail the install
• report the paths cleared to the user

Acceptance criteria

• install removes opencode-architect, opencode-architect@latest, and opencode-architect@<running version> from the resolved cache packages directory
• Older pinned versions and other packages' cache dirs are left untouched
• Pruning runs even when install is a no-op (scope already at the running version)
• Cache deletion failures warn and exit 0; the install still reports success
• Cache root honors XDG_CACHE_HOME, falling back to ~/.cache
• Unit tests cover removal targets, preserved dirs, no-op pruning, and the failure path using a temporary XDG_CACHE_HOME
• README and CHANGELOG describe the behavior

Blocked by

• None (can start immediately).