fix(download): report a filename suggestion dropped by another extension

[Yi-111-a]

Problem

Chromium lets only one extension answer chrome.downloads.onDeterminingFilename, and the API never tells the losers they lost. With a coexisting download-manager extension installed (Chrono and friends), BrowserSkill's trace-scoped suggest() at apps/extension/src/tools/download-capture.ts:236 is silently discarded:

This extension failed to name the download "BrowserSkill\tr_…\report.csv" because another extension (Chrono) has already named it "report.csv".

The download itself succeeds, and browser_path still reports the real final path, so the result looks healthy — but it implies the BrowserSkill/<trace>/ folder layout that is no longer there. Files pile up flat, repeat names get Chromium's " (1)" suffixes, and nothing anywhere tells the caller the suggestion was dropped.

Fix

Detect the drop after completion and report it as a result note, reusing the existing note convention (tool.emulate, request-help):

• download-capture.ts — suggestionWasDropped() compares the completed path against browserRelativeDir and returns suggestionDropped on DownloadCaptureResult.
• download.ts — handleDownload turns that into DOWNLOAD_SUGGESTION_DROPPED_NOTE on DownloadResult.
• crates/bsk-protocol/.../file_transfer.rs — mirrors the optional field, omitted from the wire when absent.
• Both skill bundles — a help-and-recovery.md row telling the agent to trust the reported path and ask the user to disable the other extension's renaming.

Two details worth calling out:

• Only the directory is compared. Chromium appends " (1)" to a repeated basename, which must not read as a dropped folder — there is a test pinning that case.
• A note, not an error. The file downloaded fine; only the per-call grouping was lost, and the issue explicitly says the agent still gets a usable path.

This is requested item (2) of #361, plus the troubleshooting note from item (1). Item (3) (a setting to skip the suggestion entirely) is deliberately left out — it is a separate feature surface, not a fix.

Validation

• pnpm --filter @browser-skill/extension test — 2422 passed. file-transfer.test.ts 31/31, including 3 new cases.
• cargo test --locked -p bsk-protocol — 166 passed, incl. 2 new serde tests.
• cargo clippy --locked -p bsk-protocol --all-targets and cargo fmt --check — clean.
• pnpm exec biome check . — 578 files clean.
• node scripts/check-skill-bundles.mjs — both skill directories valid.

Two pre-existing failures under full-suite parallel load (long-screenshot/exports.test.ts, human-loop.test.ts child-process timeout) were confirmed unrelated: the unmodified base fails 3 tests the same way, and both pass in isolation both with and without this branch.

Closes #361