v0.1.3 — init --force

Small UX follow-up to v0.1.2's PURPOSE chain.

revive init --force (or -f) regenerates PURPOSE from the new chain while preserving user-edited INVARIANTS and GOTCHAS. Motivated by real upgrade flow: after the chain shipped, existing .revive/static.md files still held old-extractor output, and there was no clean way to regenerate without manual rm.

Before:

$ revive init
exists: .revive/static.md (not overwriting)

After:

$ revive init --force
regenerated: .revive/static.md (INVARIANTS/GOTCHAS preserved)

Tests: 59/59 pass (+4 new cases for --force, -f, preservation, unknown-option rejection).