ci: add Windows test coverage

[erishforG]

Problem

Currently, CI only runs tests on ubuntu-latest. Windows binaries are built in the release workflow but never tested. This means Windows-specific bugs (path separators, file locking, symlinks, worktree paths) go undetected until users report them.

Current State

Job	ubuntu	macOS	Windows
cargo test	✅	-	-
cargo build	✅	✅	-
Release binary	✅	✅	✅

Proposal

Step 1: Add Windows to CI matrix (low effort, high impact)

Expand test and build jobs in ci.yml to include windows-latest:

test:
  name: Test (${{ matrix.os }})
  runs-on: ${{ matrix.os }}
  strategy:
    matrix:
      os: [ubuntu-latest, windows-latest]

build:
  strategy:
    matrix:
      os: [ubuntu-latest, macos-latest, windows-latest]

Step 2: Fix failing tests (medium effort)

Identify and fix Windows-specific issues:

• Path separator handling (/ vs \)
• File locking behavior differences (.parsec/state.json)
• Symlink permissions
• Sibling worktree layout path resolution (../repo.ticket/)
• Process signal handling

Step 3: Add Windows-specific tests (optional)

• Path handling edge cases (C:\, UNC paths)
• cfg(windows) branch coverage

Acceptance Criteria

• cargo test passes on windows-latest in CI
• cargo build --release passes on windows-latest in CI
• No regressions on Linux/macOS

Notes

• Windows runners are 2x cost vs Linux, but free for public repos
• Step 1 alone provides immediate visibility into what's broken