[TASK] discovery cache (TTL 7d, --refresh)

[kirich1409]

Context

Cache last discovery in ~/.cache/devsweep/project-roots.json with 7-day TTL; --refresh invalidates and rebuilds.

Acceptance criteria

• Cache file present after first run.
• --refresh rebuilds on demand.

Files to touch (estimate)

best-effort estimate; agent refines during implementation

Implementation notes

n/a

Dependencies

• Blocked by [TASK] discover subcommand #85

Safety checklist

See .github/ISSUE_TEMPLATE/task.yml — tick applicable invariants in the issue if any destructive path is touched.

Definition of Done

• Acceptance criteria satisfied and verifiable
• Tests added or updated
• No build / lint / test regressions
• Self-review against research-report invariants
• Code review approved