feat: speculative merge — pre-conflict simulation

[erishforG]

Vision (v1.0: Ticket lifecycle complete)

Current parsec conflicts only detects filename overlap. Simulate actual line-level merge conflicts before merging.

Proposal

parsec conflicts --simulate    # actual merge simulation

Behavior

1. In-memory merge each worktree branch against base branch
2. Report actual conflicting lines/files
3. Simulate cross-worktree merges
4. Read-only — no working directory changes

Reference

git-branchless: speculative merges — the only tool with this capability

Milestone

v1.0