Follow-up to the salvaged early-exit retry fix (#35617): the debug-browser
launch path was fire-and-forget (stderr to DEVNULL, no logging), so every
platform failure — Windows singleton forward to an existing instance, bad
profile dir, missing shared libraries, policy blocks — collapsed into the
same unactionable 'port 9222 isn't responding yet' message and debug
reports contained nothing.
- launch_chrome_debug() returns a structured ChromeDebugLaunch with
per-candidate attempts (state, exit code, stderr tail)
- browser stderr is captured to <hermes_home>/chrome-debug/launch-stderr.log
- clean exit (code 0) without the port opening is detected as Chromium's
single-instance forward and produces a targeted user hint to close all
running instances of that browser
- crash exits surface the stderr tail (e.g. missing libnspr4.so)
- every spawn/exit is logged to agent.log so hermes debug share captures it
- CLI (/browser connect) and TUI/desktop (browser.manage) both print the hint
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