[aidan] fix/web-search: nudge model to browser fallback when search backends fail

[abccodes]

a8de204: When the /api/web/search cascade exhausts every backend (DuckDuckGo throttled with no Gemini/OpenAI key or 9Router subscription), the failure text now tells the model to fall back to the in-product browser via CreateBrowserAgent instead of the dead-end "wait and retry", so a rate-limited search recovers through the real Chromium browser the product already ships.

[ciregenz]

Same story as #119: Structure lint + verify + playwright + installer are red while the identical gates pass on your other PRs, so these failures are real to this branch. Please rebase on current dev and address the lint output; re-review on green.

[ciregenz]

Substance review (separate from the CI reds): the approach is right and matches the product's honesty bar — a dead-end 'wait and retry' the model can't act on becomes a concrete CreateBrowserAgent fallback, and the real Chromium genuinely dodges the DDG throttle. Test included, good. One caveat worth a line of code or a comment: the nudge also fires in sessions where browser delegation is denied (browser tools stripped), telling the model to call a tool it doesn't have — if the shim can't see permissions, consider softening to 'if a browser tool is available'. Green-light on substance once CI is green.

[ciregenz]

Landed in eric/dev for the 1.5.5 cut (see the merge commits + follow-up fixes on eric/dev); closing as merged-via-branch.