fix(git,macos,runs): close the review findings on #22, #34 and #38

- validateRemoteUrl checks the literal scheme prefix git will receive and
  refuses %/backslash in the authority (WHATWG normalises https:/host and
  percent-escapes that git reads differently); route test for EBADURL.
- Parent-session scrub also drops AI_AGENT, and GIT_EDITOR when it is
  Claude Code's `true`.
- canonicalCwd resolves a not-yet-existing path through its deepest
  existing ancestor, so a symlinked parent still yields one lane.
- Note the clock-step ceiling on liveServers' boot-time filter.
- Docs/header wording: the Active runs count is lane-scoped.

client/src/components/run/RunHistory.tsx

@@ -14,7 +14,7 @@
  *
  * `externalSessions` (active Claude Code sessions this dashboard did NOT spawn —
  * e.g. `claude` started by hand in a terminal tab) are merged in as live rows so
- * "Active runs" counts everything actually running. They carry no tmux session
+ * "Active runs" counts everything actually running in scope. They carry no tmux session
  * the dashboard can attach to, so their only action is Resume, which spawns a
  * fresh tmux-backed `claude --resume <session>` in that cwd — Kill is withheld
  * from them for the same reason.