feat(profile): v0.6.3 — Session Shell

[g761007]

Summary

• Add skillpod shell <profile> command that spawns $SHELL with the profile env pre-loaded
• Nest guard: SKILLPOD_SHELL_DEPTH > 0 raises ProfileError("already inside a skillpod shell; exit first") → exit 1
• Child env sets SKILLPOD_ACTIVE_PROFILE, SKILLPOD_SHELL_DEPTH=1, and prefixes PS1/PROMPT with [skillpod:<profile>]
• Profile is validated via compose_effective_skillset before spawning — unknown profiles exit 1
• status JSON now includes "shell_session": {"active": bool, "depth": int} and human output shows shell session: active (depth=N) when inside a shell

Test plan

• tests/test_shell.py — 5 new tests: env building, nest guard, unknown profile, executable selection, PS1 prefix
• tests/test_cli.py — test_shell_command_nested_guard via CliRunner
• 380 passed, 1 skipped (up from 374)