Start lanes from origin's default branch and measure their diff against it #96

An agent asked to work from the latest origin/main cannot do so through the supported lane flow, and the Diff tab then reviews the lane against the wrong base.

Reproduction

1. The project checkout (~/code/ace2) is on feat/github-cache at ce1bd9b, 45 commits behind origin/main (1ab91be). Agents must not edit or switch the project checkout.
2. In a channel, the agent starts a lane with the lane tool. The tool has no base option and always runs git worktree add -b <channel>/<name> from the project's current HEAD, so the lane starts at ce1bd9b.
3. The agent runs git reset --hard origin/main in the lane and makes a 2-file, +20 change.
4. The channel's Diff tab shows image-paste-91 -> ce1bd9b with 35 files, +3580/-128, including CI and native-desktop changes that already landed on main. git diff --shortstat origin/main in the lane shows the real change: 2 files, +20.

Cause

• packages/channel/src/lanes.ts creates the worktree from the project checkout's HEAD.
• packages/channel/src/changes.ts measures a lane from merge-base HEAD <project HEAD>. When the project checkout is behind or on an unrelated feature branch, that base predates commits the lane already contains.

Expected

• Starting a lane can branch from the up-to-date remote default branch (or an explicit base) without touching the project checkout.
• The Diff tab compares a lane against the base its PR will target, so it matches the eventual PR diff.

Observed while dogfooding #91 in Ace Canary 0.0.11. Related to #28. Part of #5.