fix(agents): validate the agent slug before building the ingress URL

[tatoalo]

Problem

The agent slug from an ?agent=<slug> approval deep link becomes a subdomain label in the ingress URL (<slug>.agents.<region>.posthog.com), and it was interpolated into that template with no validation.

Deep links are untrusted input — anyone can hand a user a <scheme>://approval/...?agent=<anything> URL. A slug that isn't a plain DNS label (one containing ., /, @, #, ?, : or \) can therefore alter the structure of the resulting URL rather than just naming an agent, which is not a shape we want a credentialed client request to take.

Changes

• Adds shared slug validation so the main process and renderer use the same rules
• Rejects malformed slugs at the deep link boundary and falls back to the project-scoped Approvals inbox
• Returns null instead of building an ingress URL for invalid slugs, preserving the existing no-ingress path
• Validation exists at both the boundary and URL builder to prevent invalid state now and in future callers
• No behavior changes for valid slugs or existing callers

[trunk-io]

😎 Merged successfully - details.

[github-actions]

React Doctor found no issues in the changed files. 🎉

_{Reviewed by React Doctor for commit 797fac9.}

[greptile-apps]

_{Reviews (1): Last reviewed commit: "fix(agents): validate the agent slug bef..." | Re-trigger Greptile}