[boxcee-interview]

Closes #539

agents/bring-your-own-agent.md documented only the CLI and YAML path. Since kagent#2935 the UI authors a byo Harness directly, so a reader working in the UI had no route through the feature.

Adds a Create a BYO Harness in the UI section to the page, written as another route to the same A2A contract rather than a separate feature, as the issue asks:

• New Harness → byo under Runtime adapter; the form asks for Workload image (digest-pinned) and Command, mirroring the fields the contract table already names.
• Create harness stays disabled while the form reports a problem, so the same workload.command rule the API enforces at apply time is enforced at authoring time. Verified against harnessDraftProblems in kagent's ui/src/components/harness-form/harnessDraft.ts.
• The Harnesses tab reports the runtime as byo (runtime column in HarnessesTab.tsx).
• The model is optional on a byo agent; the Agent form asks for a model configuration only for non-byo harnesses (AgentFormPage.tsx).

Applied to both main and latest, which currently carry identical copies of this page. No matching line was needed in agent-harness.md: its workload.command row already states the requirement.

Screenshots omitted — no capture environment available here; happy to add them if you can point me at the fixture workflow.

How to Test

make build-docs (Hugo v0.160.1, the version CI pins) → HUGO_EXIT=0, 956 pages, no ERROR lines. The new section renders on both version copies with the internal ui/launch-ui link resolving.

[boxcee-interview]

CI note: all code checks pass (Static + content checks, linkChecker, DCO). The only red check is the Preview deploy — it fails at the external "Upload preview version" step (cloudflare/wrangler-action, Worker version upload) after the Hugo build succeeds, so it looks like a transient Cloudflare-side issue rather than a docs problem. A re-run of the deploy job should clear it if maintainers have the access to trigger one.