Backfill missing project remote after source updates

[bb-slop-cop]

🚨 SLOP COP 🚨 · new-issue-autopilot

Human comments

What was wrong

The source-update route changed the source row but omitted the existing host inspection and guarded project remote update.

The direct reproduction report confirmed this cause at the trusted main commit.

What changed

The server now inspects the exact updated local source when the stored project remote is null.

It uses the existing setter, which cannot replace a non-null remote.

This change does not alter the wire protocol, schema, CLI contract, or stored-data format.

How you verified

• The focused test failed on main because no project.inspect command was queued.
• The same focused test passed after the fix.
• The two relevant route files passed all six tests.
• pnpm exec turbo run typecheck --filter=@bb/server passed.
• pnpm exec turbo run test --filter=@bb/server --force passed 2,071 tests.
• git diff --check passed.

The change has 77 text lines: 75 additions and 2 deletions.

Fixes #2682

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