fix: validate sessions in cmd_status + use isolated test environment

1. cmd_status now validates session IDs against opencode session list
   - Reports 'error: base session X not found in opencode' if missing
   - Reports 'error: pm_agent session X not found in opencode' if missing

2. Test suite now uses isolated KUGETSU_DIR=/tmp/test-kugetsu-$$
   - All tests use separate test directory instead of ~/.kugetsu
   - Prevents test suite from corrupting real user data
   - Cleanup removes test directory entirely

Fixes #148

CONTRIBUTING.md

@@ -2,10 +2,10 @@
 
 ## Workflow
 
-1. Create a branch for your work: `git checkout -b fix/issue-N-name` or `git checkout -b docs/topic-name`
+1. Create a branch for your work: `git checkout -b fix/issue-N-name` or `git checkout -b feat/issue-N-feature-name`
 2. Make changes and commit with clear messages
 3. Open a Pull Request for review
-4. Do not merge directly to `master` for reviewable changes
+4. Do not merge directly to `main` for reviewable changes
 5. After approval, squash and merge
 
 ## Guidelines
@@ -17,7 +17,10 @@
 
 ## Branches
 
-- `master` — stable, reviewed content only
+- `main` — stable, reviewed content only
+- `develop` — experimental work for 0.2.x
 - `fix/*` — bug fixes
+- `feat/*` — new features
 - `docs/*` — documentation updates
+- `refactor/*` — refactoring
 - `research/*` — new research notes