[EPIC] CLI: discover + final polish (Wave 3)

[kirich1409]

Context

Wave 3 CLI — adds discover command and validates edge-case AC from cycle 2/3 reviews.

Goals

• discover interactive by default + --non-interactive --json for scripts.
• Discovery cache ~/.cache/devsweep/project-roots.json (TTL 7 days, --refresh).
• Remaining edge-case AC: AC-10, AC-12, AC-13, AC-18, AC-29.

Child tasks

Will be added as sub-issues after task batch.

Exit criteria

• All 0.1 AC pass; ready for pre-1.0 checklist.

Related documents

n/a