feat(trace): add agk trace watch to live-tail a run

[kunalkushwaha]

What

Adds agk trace watch — a lightweight, scriptable live-tail of a run's spans as they execute. It's the tail -f counterpart to the hot-reloading trace show TUI, and pairs naturally with agk run: run the agent in one terminal, watch the trace populate in another.

This is feature B5 (watch) from the FEATURES.md roadmap.

Behavior

$ agk trace watch
👀 Watching run-1782...  (Ctrl+C to stop)
────────────────────────────────────────────────────────────────
10:00:00  👤  agk.agent.run                                  2000ms
10:00:00  🤖  agk.llm.generate                               1000ms
10:00:01  🔧  agk.tool.call                                     0ms
────────────────────────────────────────────────────────────────
✅ Run complete — 3 spans, 1 LLM calls, 900 tokens, $0.0090, 2.00s
→ Inspect: agk trace view run-1782...

• No run ID → follows the live run if one is running; otherwise waits for the next run to start (e.g. from agk run).
• Ends when the run completes (its manifest.json is written) or on Ctrl+C.
• Per-span line with icons (agent / LLM / tool / workflow) + duration; final summary + trace view hint on completion.
• traceTailer reads only newly-appended complete lines, buffering any trailing partial line (so it never prints a half-written JSON record).

Testing

• go build, go vet, go test ./..., gofmt all green.
• Unit tests: tailer offset/partial-line handling, span formatting, icons, truncation.
• Verified end-to-end against a simulated live run: a background writer appended spans over ~2s then wrote the manifest; watch streamed each span and exited cleanly on completion.

Independent branch off main. Touches the README commands table / trace section (trivial conflict with the other trace PRs).

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