feat: multi-instance browser isolation for concurrent Claude Code sessions

[ca1sar]

Summary

• When multiple Claude Code windows run bb-browser commands simultaneously, they all share a single Chrome instance, causing tab/navigation conflicts
• This PR adds automatic per-session browser isolation: each Claude Code window gets its own Chrome on a dedicated CDP port
• New instances inherit login state (cookies, localStorage) from the template ~/.bb-browser/browser/user-data/ directory
• Fully backward compatible — non-session invocations (direct CLI usage) use the existing single-instance path

Design

New session-manager.ts module (self-contained, single integration point in discoverCdpPort):

• Session detection: walks process tree to find Claude Code's --session-id
• Port allocation: range 19825–19899, atomic O_EXCL file locks prevent race conditions
• Session map: ~/.bb-browser/instances/session-map.json with file-lock-protected read-modify-write
• Login state inheritance: new instances copy user-data from ~/.bb-browser/browser/user-data/ template, with --disable-cookie-encryption so cookies survive cross-profile copy
• Stale cleanup: dead Claude Code processes' sessions are auto-released on next port allocation
• Crash recovery: if a session's browser dies, it relaunches on the same port to preserve accumulated state
• Env var overrides: BB_BROWSER_PORT (explicit port) and BB_BROWSER_SESSION_ID (explicit session identity)

Files changed

File	Change
packages/cli/src/session-manager.ts	New module — all multi-instance logic
packages/cli/src/cdp-discovery.ts	3 lines added — import + call discoverSessionPort() before legacy path

Test plan

• Single Claude Code window: bb-browser open url works as before
• Two Claude Code windows: each gets independent browser, bb-browser snapshot returns different pages
• Browser crash recovery: kill Chrome, next command relaunches on same port preserving user-data
• Login state inheritance: new instance has cookies from template directory
• Direct CLI usage (outside Claude Code): falls through to legacy single-instance path
• BB_BROWSER_PORT=19830 bb-browser open url: launches on specified port
• Existing tests pass (15/15)

🤖 Generated with Claude Code