fix(#276): remove reset --hard from worktree reuse path

Problem#

WorktreeService.createWorktree() called git reset --hard origin/master in the worktree reuse path, silently destroying any agent commits already on the branch from prior phases (e.g., execute → address_review).

Task / Link#

Closes #276

Changes#

• Remove git reset --hard origin/master from the worktreeReused code path
• Keep git fetch origin to update remote refs while preserving the branch HEAD
• Update invariant comment to reflect that the branch may contain agent work
• Update unit test to assert that reset --hard is NOT called on worktree reuse

Notes#

The fresh-create and branch-attach paths are unaffected by this change.

Testing#

• Unit test 'reuses an existing valid worktree without removal, runs fetch only (no reset)' explicitly asserts resetCall is undefined
• All existing worktree tests continue to pass
• CI: Lint + Test + Build ✅, E2E Tests ✅