feat: add update command to the CLI

[joemckenney]

Summary

• Add wake update command for self-updating from GitHub releases
• Add wake llm clean command to remove orphaned models from previous versions
• Add model manifest system to track downloaded models in ~/.wake/models/manifest.toml

Changes

New commands:

• wake update - Downloads and installs latest release from GitHub
• wake update --check - Check for updates without installing
• wake llm clean - Remove orphaned/untracked model files
• wake llm clean --dry-run - Preview cleanup without deleting
• wake llm clean --force - Skip confirmation prompt

Model manifest:

• Tracks downloaded models with filename, version, SHA256, download timestamp, and URL
• Automatically records new downloads to manifest
• Used by llm clean to identify orphaned vs untracked files

Test plan

• cargo build --release succeeds
• cargo test --all passes (132 tests)
• wake update --check reports current version status
• wake llm clean --dry-run identifies orphaned models
• All new commands appear in wake --help