feat(cli): named daemon sessions for concurrent use (--session / VIBIUM_SESSION)

[paulsmith]

Summary

One-shot CLI commands proxy through a per-host daemon that was previously a singleton (fixed vibium.sock/vibium.pid/\\.\pipe\vibium), so concurrent shell scripts shared one browser and stomped on each other's page state. This PR namespaces the daemon per session:

• VIBIUM_SESSION=<name> env var (resolved in the paths package, same precedent as VIBIUM_CACHE_DIR) or the new global --session <name> flag
• Each named session gets its own socket (vibium-<name>.sock), PID file, Windows pipe (\\.\pipe\vibium-<name>), daemon process, browser, and idle timeout
• daemon status reports the session (text adds a session: line; JSON gains "session")
• Session names validated as [A-Za-z0-9_-]{1,64}; empty = default session, whose paths are byte-for-byte unchanged
• Auto-started daemon children inherit the session via the environment — no spawn-path changes needed

Usage

export VIBIUM_SESSION=checkout-tests   # or: vibium --session checkout-tests ...
vibium go https://staging.example.com/checkout
vibium click "button[type=submit]"
vibium daemon stop

New how-to guide: docs/how-to-guides/concurrent-cli-sessions.md.

Testing

• New internal/paths unit tests (default paths unchanged, session suffixing, validation)
• New tests/daemon/sessions.test.js (wired into test-daemon): two concurrent daemons with separate PIDs/sockets, stop isolation, env-var routing, session-scoped auto-start, invalid-name rejection — 5/5 green
• Suites green on this branch: go test ./..., test-cli, test-js-process, test-js-async (265), test-mcp (47), test-python (384)
• Not attributable to this branch: 7 pre-existing test-daemon failures on main (tests exercise a quit command and arg shapes the binary doesn't implement — tracked separately); test-java not run locally (no JVM on the dev host); intermittent tsup DTS EBADF build flake under Node 24 (reproduces on main)

Notes

• Known minor gap (deliberate): --session '' cannot clear an env-set VIBIUM_SESSION (use unset); flag help doesn't spell out flag-beats-env precedence
• Pre-existing (unchanged): two simultaneous auto-starts of the same session can race (os.Remove + rebind); per-session namespacing reduces real-world contention rather than adding it

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