Add a clear-cache CLI subcommand

[diegohb]

What to build

A new manual-only clear-cache subcommand that removes cached copies from OpenCode's package cache. Default targets this package; destructive broad modes require explicit confirmation.

Cache root: join(XDG_CACHE_HOME || join(homedir(), ".cache"), "opencode", "packages").

Modes:

• default (no flags): remove opencode-architect and every opencode-architect@*
• --package <name>: remove <name> and every <name>@*; requires --yes, otherwise exit 1 with a usage message
• --all: remove the entire OpenCode cache directory (the opencode directory under the cache root, not just packages/); requires --yes
• --package and --all are mutually exclusive
• reject --package values containing path separators or ..

The command is idempotent: no cached copies is a success. Deletion failures warn and continue; exit 0 unless the cache location cannot be read at all. It is never invoked automatically by any load-time hook.

Acceptance criteria

• Default clear-cache removes only this package's cache dirs
• --package <name> removes only that package's cache dirs and requires --yes
• --all removes the whole OpenCode cache dir and requires --yes
• Omitting --yes for a broad mode exits 1 without deleting anything
• --package + --all together exits 1
• Traversal-y --package values exit 1
• Running with nothing cached exits 0
• Per-mode removal failures warn and exit 0
• --help and README list the command; CHANGELOG notes it
• Unit tests cover each mode, the confirmation gate, mutual exclusion, unsafe names, and the empty-cache case

Blocked by

• Prune stale cache copies when installing #12 Prune stale cache copies when installing