[TASK] discover subcommand

[kirich1409]

Context

devsweep discover walks $HOME (bounded depth 6) with exclusion set, groups by project markers, proposes additions to projectRoots. Interactive by default; --non-interactive --json for scripted use.

Acceptance criteria

• AC-20 passes.
• Interactive prompt works; --non-interactive --json produces parseable output.

Files to touch (estimate)

best-effort estimate; agent refines during implementation

Implementation notes

n/a

Dependencies

• Blocked by [TASK] Parallel walk across projectRoots (bounded 4-8) #59
• Blocked by [TASK] Exclusion split (basename + pathPrefix with canonicalize) #60
• Blocked by [TASK] main.swift composition root #66

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