fix: accept alphanumeric Linear issue team keys

[schpetbot]

Summary

• centralize Linear issue identifier parsing in a shared utility
• allow linear issue update to infer team keys from alphanumeric identifiers like PLA4-16916
• reuse the same parsing logic for jj trailers and git branch issue inference

Testing

• deno check src/main.ts
• deno lint
• deno task test

Fixes #193