Load an agent from a template zip in the app

[mmabrouk]

🤖 The AI agent says:

Context: The templates work (PRs #7169, #7173, #7170, #7174, #7171, #7172, being consolidated into one PR) lets a user save an agent as a template zip from chat ("Save as template (.zip)") and loads zips through the API: POST /api/agent-templates/validate and POST /api/agent-templates/load with an upload or session_file source, a version + digest pin and an Idempotency-Key. There is no screen in the app to load a zip, so a recipient cannot use a zip they received without calling the API. GitHub folder loading has the same gap (API only; the web client's source type has no github kind).

What to build: in the new-agent / template flow (both the default /m app and classic /w), let the user pick a zip (upload) or a zip in the session's files, call validate, show any issues (file, field, next step), and on a valid result call load with the returned version and digest pin, then open the new agent. Optionally the same for a public GitHub repo + full commit SHA + folder.

References: spec openspec/changes/create-share-load-templates/specs/app-template-gallery/spec.md and specs/template-upload-loading/spec.md; API router api/oss/src/apis/fastapi/agent_templates/router.py; web client web/packages/agenta-entities/src/workflow/api/agentTemplates.ts (already has the source union + validate client).

Acceptance: a zip exported with "Save as template" loads into a working agent from the UI with no API calls; invalid zips show the validation issues; retrying does not create duplicates.

[linear-code]

AGE-4467

[bhattyuvraj22]

Hi @ashrafchowdury! I'd like to work on this issue, if it's available for contributors. I see it's assigned to you. If you're already working on it, no problem, just let me know.

I saw that #7206 merged into release/v0.121.5, so I'd base my work on that branch unless you prefer main.

Planned approach

1. Add a "load from zip" entry point in the new-agent/template flow, in both the default /m app and classic /w, sharing logic through agenta-entities where possible.
2. Let the user pick a zip by upload or from the session's files.
   3.Call POST /agent-templates/validate and show any issues (file, field, next step) in the UI. Loading stays blocked until the package is valid.
3. On a valid result, call POST /agent-templates/load with the returned version and digest pin. I'll send one Idempotency-Key per load attempt and reuse it on retries, so retrying doesn't create duplicates. Then open the new agent.
4. Optional: the same flow for a public GitHub repo + full commit SHA + folder. This needs a github kind added to the web source type, so I can do it in this PR or as a follow-up, whichever you prefer.

Two questions:

Is there a preferred place in the UI for the entry point (for example next to the template gallery, or in the new-agent dialog)?
Should GitHub loading be in scope here, or a separate issue?

I'll wait for your go-ahead before starting.