feat: package manifest history and rollback in the dashboard

[paddo]

Summary

• Packages tab gains a manifest history view: h opens a manager's commit history, Enter on an entry shows the diff — mirroring the Files tab.
• New tether rollback packages <manager> <commit> command rolls a package manager's installed set back to a manifest snapshot: it computes the reverse delta (install what the snapshot had, uninstall what was added since), applies it, then re-syncs so removals are recorded and the union manifest updates.
• Dashboard: R on a history entry opens a confirmation modal with +install / −uninstall counts, then spawns the rollback command as a subprocess (same pattern as sync).

Scope / limitations

• Brew is not covered. Its Brewfile format and cask/tap handling need a separate pass; R on a brew entry reports "not yet supported". History viewing still works for brew.
• Fleet behaviour. Rolling back affects this machine and records removals so they don't propagate back to it; it does not force-uninstall on other machines (the package model is descriptive, not declarative).
• Projects rollback is not included (would require a new Projects tab).

Test plan

• Packages tab: h lists a manager's manifest history, Enter shows the diff
• R on a history entry shows the confirmation modal with correct install/uninstall counts
• Confirming runs tether rollback packages and the manager's installed set matches the snapshot afterward
• tether rollback packages <manager> <commit> works standalone from the CLI
• Brew history entries report rollback is unsupported rather than erroring