fix(auth): populate profiles.github_handle at signup and sync on update (GHB-110)

[aagear]

Fixes #110

Problem

Issue #110 reports that profiles.wallet_pubkey and profiles.github_handle are null for newly registered dev accounts, blocking MCP submissions.create and whoami.

Existing PRs (#133, #139, #140) address wallet_pubkey backfill during delegation, but none fix github_handle -- the profiles table never gets the GitHub handle even though the developer signup form captures it.

Root Cause

In frontend/lib/auth-privy.tsx:

1. persistDevRow: The profiles table insert omits github_handle entirely. The developers table gets github_handle, but profiles does not.

2. updateUser: When a dev updates their GitHub handle, only the developers table is updated -- profiles.github_handle is never synced.

Changes

1. persistDevRow -- populate profiles.github_handle on signup

Add github_handle: data.github ?? null to the profiles insert. The data is already available from the onboarding form.

2. updateUser -- sync github_handle to profiles on update

Extend profilePatch to include github_handle. When a dev changes their GitHub handle, both profiles and developers tables are updated.

Why this is the root-cause fix

The MCP auth path reads profiles.github_handle and profiles.wallet_pubkey. Existing PRs fix wallet_pubkey reactively during delegation. This PR fixes github_handle proactively during signup so the profile row is correct from the start, not patched later.

Testing

• pnpm typecheck -- no new type errors (profiles table already has github_handle column in db.types.ts)
• No behavioral change for company accounts (they do not have a GitHub handle)
• Visual output unchanged

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