Feat: show agent context attachments

[moonlight16]

Summary

Display attached context resources in rossoctl agents get and improve the human-readable output of rossoctl context get.

Agent details

When the server returns context attachment metadata, human output now includes a Contexts section showing:

• Context name
• Type
• Mount path
• Access mode
• Backing PVC claim

--json preserves the additive contexts field.

Older servers and agents without recorded context metadata continue to work without displaying the section.

Context details

Replace the terse single-line context get output with labeled sections:

• Context Information
• Storage
• Attachment

Context Information
  Name:       research
  Namespace:  team1
  Type:       workspace
  Status:     ready

Storage
  Backend:        pvc
  Size:           1Gi
  Access Mode:    ReadWriteOnce
  Storage Class:  <default>

Attachment
  Kind:   pvc
  Claim:  context-research

Validation

• CLI command and API model tests pass.
• Live-tested human and JSON agent output against the updated Rosso backend.
• Verified labeled context get output against a live context.
• Verified agents without contexts remain compatible.

Dependency

Requires rossoctl/rossoctl#2447 to populate context attachment metadata. The CLI remains backward-compatible with older servers.