fix(browser): R3 — restore fs cat root, daemon lane capture, grep hyphen guard

Address @yuh-yang's three R3 blockers on PR #308. All three land in this PR;
the grep parser issue (blocker 3) takes Path B — Python-side rejection with
a clear error message — while the real parseArgs `--` work goes upstream to
a future DOMShell release.

  1. fs cat at root no longer raises Python ValueError before reaching
     DOMShell. The absolute-root guard added in round 5 was overreaching —
     it converted DOMShell's standard "Usage: cat <name>" error into a
     Python exception. Guard removed; cat '' now reaches the kernel and
     surfaces DOMShell's own error string, restoring pre-migration
     behavior. Stale "should raise" tests flipped to "should not raise +
     return parseable error result".

  2. Daemon-mode/no-session lane handling is now consistent across daemon-
     alive and daemon-dead paths. The previous "shared" branch (commit
     99d1182) claimed per-connection-default stickiness that doesn't hold
     when _daemon_session is None and each call spawns a fresh
     ClientSession. Replaced with a module-level _daemon_lane_id captured
     on the first daemon-no-session call and reused on every subsequent
     call. Works in both paths: alive daemon reuses the id on its
     persistent connection; dead daemon's fresh spawns swapToAgentLane()
     into the existing Chrome tab-group by id (groups persist across
     MCP session boundaries). Stale-lane failure mode (user closes the
     group manually) propagates DOMShell's own error — same shape as the
     session-bearing path. Replaces the previous-round "shared" test
     with first-call-captures + subsequent-calls-reuse tests.

  3. Hyphen-prefixed grep patterns now raise a clear Python-side
     ValueError instead of silently failing at DOMShell. The current
     DOMShell parseArgs treats any arg starting with "-" as a flag —
     no "--" separator, no -e <pattern> form — so `grep -r -- -foo`
     and `grep -r -e -foo` both fail at the kernel. Tracked upstream as
     a parser-limitation issue; the real fix ships with the next
     DOMShell release that already justifies a Chrome Web Store
     submission. Until then, the wrapper rejects the input with
     guidance instead of letting it silently fall through.

191 passing locally (was 189 + 4 new tests - 1 deleted shared-lane test -
1 old cat-root test replaced).

Cc @yuh-yang

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