feat(display): add final agent summary to completion output

[ssmith-pc]

Summary

• Renders the completing run's plain-text response as a new Summary block in rings run completion output, between the Budget gauge and the Audit logs line.
• Word-wrapped at 76 columns, truncated to 10 lines. When truncated, the trailing … line appends a dim (full response in runs/NNN.log) pointer so the complete text is one directory lookup away.
• Best-effort: if the log cannot be read or the response is empty, the block is silently omitted. Human output mode only — JSONL already emits a structured summary event.

Example:

   Budget      █████████░░░░░░░░░░░  $4.54 / $10.00  (45%)

   Summary     **Summary of what's on the branch:**

               The fix replaces the boolean return with a `ContainerStatus` enum
               ...
               …  (full response in runs/004.log)

   Audit logs  /home/user/.local/share/rings/runs/run_xyz/

Test plan

• cargo test --features testing --lib — 343 passed (7 new tests covering empty input, label/indent, truncation, wrap width, hint rendering, and hint suppression when not truncated)
• cargo fmt --check clean
• Manual smoke test of format_completion output (short + long summary)
• Rebuild and run end-to-end against a real workflow to confirm the block renders from a live audit log

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