[Phase 3] Implement or explicitly defer cage run --continue session resume

[Saber5656]

Background

PRD §11.5 defines cage run --continue (resume the previous conversation in the same directory, modeled on claude-docker --continue), and the CLI UX spec lists the flag. No current issue covers it. Session persistence (#13) provides the metadata and volumes needed, so the gap is the resume flow itself. Leaving the flag undefined risks another parse-only flag like the ones that shipped in the legacy CLI.

Scope

• Decide the pre-alpha stance: implement a minimal resume, or defer with a clear unimplemented error (either is acceptable; it must be explicit).
• If implemented:
  • Resolve the latest session for the project (reusing [Phase 2] Persist session workspaces and the Cage baseline ref #13 resolution rules, including the ambiguity handling for concurrent sessions).
  • Restart the stopped container (or recreate it from the persisted workspace/git volumes when the container was removed).
  • Re-inject credentials — they were deleted after the first start per FR-2.6, so resume needs the full injection path again with the same guarantees ([Phase 1] Inject credentials through tmpfs and delete them after startup #11).
  • Pass the agent's own continue/resume flag (e.g. claude --continue) through the adapter.
• Define interaction with --cleanup (a cleaned-up session cannot be resumed) and with crashed sessions (resume vs sync --from-volume recovery).

Acceptance Criteria

• cage run --continue either resumes the latest session or fails with an explicit, documented unimplemented error — never a silent no-op.
• When implemented, credential re-injection on resume keeps the same security guarantees as first start.
• --continue with no prior session yields a clear error, not a panic.
• Interaction with --cleanup and crashed sessions is documented.

References

• cage-demo/docs/PRD.md §11.5 (additional CLI commands)
• cage-demo/docs/cli-ux/cli-ux-spec.md §3 (--continue)
• cage-demo/src/session/ SessionStatus lifecycle
• [Phase 1] Inject credentials through tmpfs and delete them after startup #11 (credential injection), [Phase 2] Persist session workspaces and the Cage baseline ref #13 (session persistence), [Phase 2] Implement --from-volume sync and diff for crash recovery #16 (crash recovery)