fix: chore: remove dead tmux-based session code (agent-session.sh, phase-handler.sh) (#262)
- Delete lib/agent-session.sh (entirely dead file with no active callers)
- Delete dev/phase-handler.sh (entirely dead file with no active callers)
- Update lib/formula-session.sh to remove tmux-based functions:
- Removed: start_formula_session, run_formula_and_monitor, formula_phase_callback,
write_compact_context, remove_formula_worktree, cleanup_stale_crashed_worktrees
- Kept utility functions: acquire_cron_lock, check_memory, load_formula,
profile_write_journal, formula_prepare_profile_context, build_graph_section, etc.
- Update dev/phase-test.sh to inline read_phase() function (no longer sources agent-session.sh)
- Update documentation: AGENTS.md, lib/AGENTS.md, dev/AGENTS.md, .woodpecker/agent-smoke.sh,
docs/PHASE-PROTOCOL.md, lib/pr-lifecycle.sh
- All 38 phase tests pass
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