feat(memory): split remote scans into ours and theirs

[patrick-ogrady]

Summary

Remote scans now retrieve the authenticated namespace and all other visible namespaces independently, then present both groups to the agent as ours and theirs. This prevents either source from crowding out the other while leaving the agent free to choose the strongest guidance.

• make each store scan operate on one namespace scope and return a candidate vector; an empty vector means no match
• issue own and others requests sequentially on one bookmark chain, with each response independently ranked and bounded
• derive scan scopes from the authenticated server identity and validate response ownership and ordering
• push scope filtering and a configurable per-scope result limit into the Cloudflare Worker
• render only the current grouped scan shape in the TUI and update the memory documentation

Local scans continue to use the selected local backend and populate only ours. Remote failures remain all-or-error and never fall back locally.

[codspeed-hq]

Merging this PR will not alter performance

✅ 18 untouched benchmarks
⏩ 31 skipped benchmarks¹

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_{Comparing prefer-our-memory (6926bc8) with main (ee5b1ca)}

Footnotes

1. 31 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩