Require Linear In Progress claim before agent spawn thrash

[TheGreatAxios]

Summary

• Agents claim a Linear issue as In Progress with mcp__linear__save_issue before worktree, explore, or build thrash
• Parallel lanes claim only their own issue IDs; claim failures are reported instead of pretended

Verification

• bun run typecheck and bun run build pass via pre-commit
• Skill and Skywalker package text place the claim as a hard first step

Fixes CL-7054

[linear-code]

CL-7054