feat(traces): complete CLI trace contract

[mrgoonie]

Summary

• Align traces list/get/follow/export with current GoClaw server trace contracts.
• Add traces timeline <runID> for GET /v1/runs/{runID}/timeline.
• Refresh trace fixtures/tests to server-shaped payloads and update docs/plans.

Details

• traces list now preserves {traces,total,limit,offset} in JSON/YAML and renders rows from traces in table mode.
• traces list supports server filters: --agent, --user, --session-key, --status, --channel, --limit, --offset.
• Deprecated unsupported list flags --since and --root-only are hidden and rejected with validation errors.
• traces get renders {trace,spans} with id, parent_span_id, and span_type.
• traces export validates IDs and path-escapes route params before HTTP.
• traces follow remains one-shot and shares server-field table rendering.
• No traces replay command added because server dev does not expose replay.

Validation

• /usr/local/go/bin/go test -count=1 ./cmd -run 'TestTraces(List|Get|Follow|Timeline|Export)'
• /usr/local/go/bin/go test -count=1 ./cmd ./internal/client ./internal/output
• /usr/local/go/bin/go test -count=1 ./...
• /usr/local/go/bin/go vet ./...
• /usr/local/go/bin/go build ./...
• git diff --check

Notes

• Local shell did not have go on PATH; validation used /usr/local/go/bin/go (go1.25.3 darwin/arm64).
• Supersedes the older flat-fixture trace detail plan for implementation details.

Closes #17.