Add more source adapters (Linear, Azure DevOps, Jenkins, CircleCI, Bitbucket)

[NewerKey]

Only Jira, GitLab, and GitHub are built in today. The README's own
"Status & contributing" section invites adapters for these tools, and
now that keythread_adapters entry-point plugin discovery exists
(src/keythread/adapters/registry.py::discover_adapters), a new adapter
no longer needs a PR into this repo — it can ship as its own installable
package.

Suggested starting point: Linear — clean webhook model, real demand,
and a good first real-world exercise of the entry-point mechanism
end-to-end (not just the unit tests added for it).

Reference:

• adapters/github.py + tests/adapters/test_github.py — the coverage
  bar a new adapter is expected to meet.
• README → "Writing a new adapter" for both the explicit-registration and
  entry-point-publishing paths.