feat(extension): hybrid search + surface document chunks in auto-recall

[HaoChiBao]

Summary

Auto-recall in the browser extension was leaving useful document context on the floor.

1. /v4/search was called without searchMode, so it used the API default ("memories").
2. Hits were formatted as `${result.memory}` only. Hybrid/document hits that only set chunk became the literal string undefined in the Included Memories prompt.

This PR:

• Requests searchMode: "hybrid" (same recommendation as the public docs / VoltAgent path in @supermemory/tools)
• Formats each hit as memory || chunk, skips empties, and renumbers contiguously
• Adds an offline A/B eval harness + proof doc so the gap and the fix stay regression-tested

Does not change server ranking. It only improves what the extension injects when the API already returns chunks.

Why this is not already upstream

Checked against current main and open extension PRs:

• apps/browser-extension/utils/search-request.ts on main still has no searchMode
• background.ts on main still pushes result.memory only
• Overlapping open PRs (fix(extension): stop fragmenting Included Memories that contain commas or newlines #1339 / fix(extension): finish Included Memories leftovers on T3 (#1257) #1421) fix Included Memories popup lifecycle / comma split, not search request shaping

Proof

cd apps/browser-extension
bun run eval:hybrid-recall
# or
bun test utils/hybrid-recall.eval.test.ts utils/search-request.test.ts

Latest offline A/B on 4 gold fixtures:

Metric	Legacy (memory only)	Next (hybrid + memory || chunk)
undefined prompt lines	6	0
Chunk-only texts recovered	0/4	4/4
All expected texts recovered	6/10	10/10

Example (chunk-only-page):

Legacy	Next
1. undefined	Incident runbook…
2. undefined	Rollback: redeploy…

Full write-up: apps/browser-extension/docs/hybrid-recall-eval.md

Test plan

• bun test utils/hybrid-recall.eval.test.ts utils/search-request.test.ts (9 pass)
• bun run eval:hybrid-recall → PASS
• Rebased onto latest main; no overlap with fix(extension): stop fragmenting Included Memories that contain commas or newlines #1339 / fix(extension): finish Included Memories leftovers on T3 (#1257) #1421 search bodies
• Optional smoke: build extension, save a note + a preference, confirm Included Memories shows doc wording and no undefined rows