Add Herdr agent runtime

[codeaholicguy]

Summary

• add global agent runtime selection with tmux as default and Herdr as the MVP alternate provider
• store runtime and opaque runtime refs on existing agent registry rows while preserving legacy tmux behavior
• route agent start/send/wait/open/kill through Herdr when the stored runtime is Herdr-backed
• add lifecycle docs and focused coverage for config, registry, runtime dispatch, and Herdr CLI behavior

Validation

• Commit hook secret scan passed
• Commit hook full workspace lint passed
• Commit hook full workspace tests passed: 96 files, 1165 tests
• Focused checks run before commit: agent-manager typecheck, CLI typecheck, agent-manager build, feature lifecycle lint, diff whitespace check

Risks

• Real Herdr behavior still depends on the local Herdr CLI/API command contract.
• Security review noted raw Herdr terminal output should get a sanitizer in a follow-up; intentionally not included in this commit.