feat(metadata): support Space URL appData

[neekolas]

Summary

• Add optional ConversationCustomMetadata.spaceUrl at protobuf field 9.
• Read and write the field through the existing appData codec with last-write-wins behavior.
• Verify the wire field, an iOS-generated vector, and safe handling of the deprecated unknown connections field.
• Include a patch Changeset for the downstream Herald release.

This is the codec contract Herald Lite and assistant creation will use in follow-up PRs. It does not change appData compression behavior.

Validation

• pnpm exec vitest run test/utils/metadata.test.ts — 45 passed
• pnpm typecheck
• pnpm build
• git diff --check

[macroscopeapp]

Approvability

Verdict: Approved 4835ad7

This PR adds a new optional spaceUrl field to the conversation metadata protobuf schema. The change is purely additive - existing serialization/deserialization behavior is unchanged, and optional protobuf fields are backward compatible by design. Comprehensive tests verify correct encoding and round-trip behavior.

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