feat: add Omnigent routing runtime support

[smota]

Epic: #27

Summary

Add Omnigent as a supported optional routing/runtime option for role routing.

Acceptance criteria

• Supported routing slugs include omnigent.
• Route config validation accepts Omnigent owners/fallbacks.
• Route resolution can select Omnigent and return a handover doc path.
• docs/agents/omnigent-routing.md documents when to use Omnigent vs direct agent CLIs.
• Tests cover Omnigent route validation and resolution.

Test plan

• Run pnpm test.
• Run pnpm test:workflow.
• Run pnpm format:check.
• Validate example routing config with Omnigent.

Workflow classification

• Type: feature / dx / tooling / documentation
• Suggested profile: standard
• Default execution: single-agent

[smota]

Corrected issue labels after provenance review:

• replaced incorrect drafted-by:claude with drafted-by:pi because these issues were created by the current Pi runtime;
• removed over-applied documentation where documentation is a supporting deliverable rather than the primary work;
• left documentation on chore: document Omnigent-powered SDLC scenarios #31 because that child issue is documentation-first.

Follow-up structural fix opened as #34.

[github-actions]

Integrated into development by PR #35: https://github.com/smota/multi-agent-sdlc/pull/35

Merge commit: 2866f00a11e2d38d291c0b440cdd521ec8c78f81
Release/promotion to main is tracked separately.