fix: prevent claim prompts from tracking main

[atomantic]

Summary

• Four shipped task-prompt defaults (plan-task, claim-issue, claim-issue-gitlab, claim-issue-jira) told agents to run git worktree add -b <branch> <path> origin/main, which makes the new branch inherit main as its git upstream. A later config-derived push (as used by /do:pr-style flows) could then land straight on main instead of opening a PR — the exact CoS agent worktree branches track refs/heads/main, so /do:pr's config-derived push lands on main #4172 failure mode, but on the prompt path rather than the code path.
• Adds --no-track to those four git worktree add commands. Each prompt's later git push -u origin <branch> step still sets the correct upstream, so untracked is the right shape. dependency-updates is deliberately left alone — it branches from a PR's own head, where inheriting that branch as upstream is correct.
• Bumps PROMPT_VERSIONS for all four touched prompts, preserves each pre-change body in PREVIOUS_DEFAULT_PROMPTS so installs holding the old default auto-upgrade instead of being flagged customized, and regenerates the integrity snapshot.
• Adds a source-grep guard test asserting every worktree add command in these four prompts carries --no-track, plus a companion test confirming dependency-updates still tracks its PR head.

Closes #4185.

Test plan

• cd server && NODE_ENV=test npx vitest run taskPromptDefaults — 37/37 passing
• Full server suite run under heavy concurrent-worktree CPU contention showed unrelated Test timed out in 10000ms flakes in a different, non-overlapping set of files on each run (peerSync/privacy/sprites/universeBuilder on one run, imessageSync and others on another) — consistent with resource-contention flakiness, not a regression from this change