feat: parsec test — parallel test execution across worktrees with caching

[erishforG]

Vision (v1.0: Ticket lifecycle complete)

Run tests in parallel across worktrees with result caching.

Proposal

parsec test [ticket]             # run tests in current worktree
parsec test --all --jobs 4       # parallel tests across all worktrees
parsec test --all --cache        # tree-hash-based caching

Config

[test]
command = \"cargo test\"
jobs = 4
cache = true

Behavior

• Independent test execution per worktree (leverages physical isolation)
• Commit tree hash caching — skip re-run if unchanged
• Clear failure reporting per worktree

Reference

git-branchless: git test --strategy worktree --jobs N + tree hash caching

Milestone

v1.0