feat(ai): add agent-roster detection script with testsHEADmain

agent-roster is the single detection primitive for concurrent same-project Claude sessions: pgrep -x claude, resolve each pid's cwd from /proc, keep those at or inside the project root, and drop the scanner's own ancestry. It exits 0 when alone, 1 when other agents are present (printed pid + cwd), and 2 when the roster can't run. Both the helper launcher and the in-session startup check will call this rather than re-scanning.

pgrep and /proc are the system boundary, so I made them injectable (ROSTER_PGREP, ROSTER_PROC, ROSTER_SELF_PID) and the bats exercise the real include/exclude logic against fixtures, no agents spawned. The unavailable paths (no /proc, or pgrep absent) report on stderr and exit 2 rather than a false "alone".

This is the first slice of the helper-instance task. Startup and ai --helper wiring follow.

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