models: cache weights outside ~/Documents so the LaunchAgent can load them

[bdeol2]

parrot install --launch-at-login cannot start on a clean machine. The agent
loads the model, fails, exits 1, and — because the generated plist sets
KeepAlive.SuccessfulExit=false — launchd restarts it in a loop.

Cause

WhisperKit delegates downloads to HubApi, whose downloadBase defaults to
~/Documents/huggingface. ~/Documents is TCC-protected on macOS:

• Run from a terminal, parrot inherits the terminal's Documents access, so
  model loading works. This is why it isn't visible in normal foreground use.
• Run by launchd, parrot has no Documents access of its own, so warmUp()
  fails.

The error also misreports what happened:

warmup failed: modelsUnavailable("Model not found. Please check the model
or repo name and try again.\nError: invalidMetadataError("Could not remove
corrupted metadata file: “model.mil.metadata” couldn’t be removed because
you don’t have permission to access it.")")

Nothing is corrupt. The directory is unreadable, and HubApi surfaces that as a
metadata error while trying to clean up.

Fix

Pass an explicit downloadBase of ~/Library/Application Support/parrot,
which is outside TCC. I chose this over granting parrot access to Documents or
Full Disk Access because it needs fewer privileges, not more — the daemon
already holds Accessibility and Microphone, and adding a third broad grant to
reach a cache directory seemed like the wrong direction.

Existing users

Weights are kept by moving the cache once, rather than re-downloading:

mkdir -p ~/Library/Application\ Support/parrot
mv ~/Documents/huggingface/models ~/Library/Application\ Support/parrot/models

Worth a README note; happy to add one here if you'd like it in the same PR.

Verified

• Before the change, under launchd: the invalidMetadataError above, runs
  incrementing on a loop.
• After, under launchd: ✓ whisper-base.en ready, then listening on fn hold,
  runs = 1, no restarts. Dictation into a native text field works end to end,
  with the migrated cache and no re-download.
• Foreground use is unaffected: parrot models download whisper-base.en
  resolves from the new location.
• swift build -c release clean on this commit alone.

Not verified: only tested with whisper-base.en on macOS 15 / Apple Silicon,
and only with a migrated cache — I did not test a first-run download into the
new location from scratch.

Relationship to #9

Complementary, no overlap. #9 hardens where the daemon writes its logs; this
fixes the daemon being unable to start at all. Different files, so no conflict
— #9 touches Install.swift/Parrot.swift, this only touches
WhisperKitTranscriber.swift.