feat(capabilities): dual-mode deployment — agent-per-node + squad-per-pod (#514)

[tamirdresher]

Summary

Extends PR #520's capability routing to support both deployment modes:

Mode	Env Var	Manifest	Use Case
agent-per-node (default)	none needed	.squad/machine-capabilities.json	One Ralph per K8s node
squad-per-pod	SQUAD_POD_ID=pod-name	.squad/machine-capabilities-{podId}.json	Full Squad instance per pod

Changes

• capabilities.ts: Pod-specific manifest loading with fallback chain
• capabilities.test.ts: 6 new tests for dual-mode behavior
• machine-capabilities.md: Updated docs with K8s downward API examples

Fallback chain (when SQUAD_POD_ID is set)

1. .squad/machine-capabilities-{podId}.json (pod-specific)
2. .squad/machine-capabilities.json (shared)
3. ~/.squad/machine-capabilities.json (user home)
4. null (opt-in - all issues pass through)

K8s Integration

env:
  - name: SQUAD_POD_ID
    valueFrom:
      fieldRef:
        fieldPath: metadata.name
  - name: SQUAD_DEPLOYMENT_MODE
    value: squad-per-pod

Closes #514