v0.3.0: Sessions & Cobra CLI

TL;DR

• Session tracking — agentjail sessions list shows active and past agent sessions with PID-based detection.
• Cobra CLI — migrated to Cobra for automatic --help, shell completions, and subcommand groups.
• Platform procwalk — process tree walking split into _darwin.go / _linux.go with build tags.

Added

• agentjail sessions list — active and past agent sessions with PID-based active detection via process tree walking
• Session names from Claude Code metadata — sessions display human-readable names instead of opaque IDs
• Daemon session tracking wired into CLI dispatch
• SQLite session store (internal/store) with schema, queries, and models
• Cobra CLI framework — automatic --help generation and shell completions

Fixed

• Platform-specific procwalk — split into _darwin.go (sysctl) and _linux.go (/proc) with //go:build constraints
• Testable active sessions loader with parity sync
• Added ListSessionsFiltered to mock store

Changed

• AGENTS.md — added build-tag rule for platform-specific code