v0.3.0 — premises, preflight, and a merge locked to its OK

• wave why <issue>: the premises behind READY / NOT READY (leaf, open, blockers closed, no PR, no worktree) with their evidence, and the conclusion.
• wave doctor: preflight — gh login, git ≥ 2.38, tracked changes in the main checkout, real gate, protected paths, unmerged worktrees without a PR, leftovers, disk for the batch. dispatch runs it and refuses on ✗; it also refuses an issue that is not READY.
• wave order --plan p.json + wave merge --plan p.json --yes: the plan pins the base SHA and every PR head; if any moved since the OK, no merge. A PR whose issue still has an open blocker, or that closes a parent, is refused.
• wave lint reports contradictions between text and data (Depends on without blocked_by, handoff branch without the issue number, Closes of another issue, a parent with a handoff block).
• States are exhaustive and exclusive, asserted in code.