fix(agent-manager): treat only ESRCH as definitive process death

[codeaholicguy]

Summary

• treat only ESRCH from process.kill(pid, 0) as definitive process death
• preserve registry custom names and tmux metadata on EPERM and indeterminate probe failures
• keep register and rename name-conflict handling conservative through the shared liveness predicate
• add deterministic regressions for EPERM, ESRCH, unknown failures, conflict behavior, and two refresh cycles

Validation

• npm test --workspace @ai-devkit/agent-manager -- AgentRegistry.test.ts AgentManager.test.ts (71 passed)
• npm test --workspace @ai-devkit/agent-manager (518 passed)
• npm test --workspace ai-devkit -- agent.service.test.ts agent.test.ts (104 passed)
• npm run typecheck --workspace @ai-devkit/agent-manager
• npm run lint --workspace @ai-devkit/agent-manager
• npm run build --workspace @ai-devkit/agent-manager
• npx ai-devkit@latest lint
• mutation gate: old catch-all-dead behavior failed 6 targeted tests; restored fix passed all 71

Risks

• Conservative unknown-probe handling can retain a stale row when the platform never returns ESRCH. This is intentional to avoid destructive pruning without definitive death evidence.
• Existing API and SQLite schema are unchanged.