Add pre-commit hook installation to onboarding and upgrade processes

- prompts/onboarding.md: Step 2c installs git pre-commit hook for new projects
- scripts/upgrade.sh: Installs pre-commit hook during upgrade (with symlink detection)
- scripts/install.sh: Mentions hook installation in post-install next steps

prompts/onboarding.md

@@ -66,6 +66,15 @@ Verify that the project is set up for v2.0 state enforcement:
 3. If the project has existing tasks, run `python ~/.automaton/scripts/status.py --audit --project {project}` to check for violations or manually created tasks that need `.state` files.
 4. If the project has existing tasks without `.state` files, run `python ~/.automaton/scripts/status.py --upgrade --project {project}` to bootstrap `.state` files from artifact heuristics.
 
+### Step 2c: Pre-Commit Hook Installation
+
+Install the automaton pre-commit hook to block commits when no task is in an edit-allowed phase:
+1. Check if `{project}/.git/hooks/` exists (the project uses git).
+2. If it does, create a symlink: `ln -sf ~/.automaton/scripts/git-hooks/pre-commit {project}/.git/hooks/pre-commit`
+3. If `{project}/.git/hooks/pre-commit` already exists and is not our symlink, warn the user and ask whether to replace it.
+4. Verify the hook: `python ~/.automaton/scripts/status.py --can-edit --project {project}` should return exit code 1 (DENIED) since no tasks exist yet.
+5. Note in the onboarding report whether the hook was installed.
+
 ## Output
 
 Create or update the following inside {project}/.automaton/: