fix: bound Obsidian CLI capability probes (#250)

[levineam]

Summary

• run Obsidian capability probes in an owned worker and process group
• contain timed-out descendants through TERM/KILL escalation without touching unrelated processes
• preserve indeterminate/busy timeout classification and bound captured output by bytes
• gate the regression in the root test suite, including POSIX descendant and Windows containment seams

Verification

• focused adapter suite: 27/27 passed
• full dependency-provisioned npm test: passed
• independent Terra review: no P0-P2 findings
• package dry run includes the worker

Safety

No real Obsidian process or vault was invoked or written. The regressions use temporary fake CLI fixtures only.

Closes #250

[levineam]

HOLD — final independent review found an identity-unsafe cached-PID kill path and capture/polling regressions at exact head fc9a7f2. Do not merge. The portable process-group approach cannot guarantee arbitrary detached-descendant containment without risking unrelated processes; redesign under a separate bounded contract. See #250 and #260.