Fix task auto-commit with pre-commit hook

Implemented proper pre-commit hook that: - Calls 'task export --flush'
to consolidate tasks - Auto-stages .tasks/tasks.jsonl if modified -
Runs before every commit

Added reminder message after 'task update' to inform users that task
changes will be committed on next git commit.

Updated AGENTS.md to document the auto-commit behavior.

This fixes the bug where task status updates (e.g., marking tasks as
Done) were not being committed to git.

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