feat(browse): --navigate flag on download for browser-triggered files

Adds the --navigate strategy from community PR #1355 (originally from
@garrytan-agents). When set, download navigates to the URL with
waitUntil:'commit' and captures the resulting browser download via
page.waitForEvent('download'), then saves via download.saveAs().
Handles URLs that trigger files via Content-Disposition headers,
multi-hop CDN redirects requiring browser cookies, or anti-bot CDN
chains where page.request.fetch() can't follow the auth/redirect
chain.

Defaults still use the existing direct-fetch strategy. --navigate is
opt-in.

Goes through the same validateNavigationUrl SSRF gate as goto, so
download --navigate cannot reach IPv4 metadata endpoints (AWS IMDSv1,
GCP/Azure equivalents) or arbitrary internal hosts.

Inferred content type from suggested filename for common extensions
(epub, pdf, zip, gz, mp3/mp4, jpg/jpeg/png, txt, html, json) — falls
back to application/octet-stream. Same 200MB cap as Strategy 1.

Frames the use case generically (anti-bot CDN, Content-Disposition,
redirect chains) rather than naming any specific site, per project
voice rules.

Co-Authored-By: @garrytan-agents
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

gstack/llms.txt

@@ -64,7 +64,7 @@ Run with `browse <command> [args]`. Full reference: `browse/SKILL.md`.
 
 ### Extraction
 - `archive [path]`: Save complete page as MHTML via CDP
-- `download <url|@ref> [path] [--base64]`: Download URL or media element to disk using browser cookies
+- `download <url|@ref> [path] [--base64] [--navigate]`: Download URL or media element to disk using browser cookies.
 - `scrape <images|videos|media> [--selector sel] [--dir path] [--limit N]`: Bulk download all media from page.
 
 ### Inspection