Support Linear Action

[samgaw]

Mirrors the jira action to add support for Linear issue tracker.

The action config block looks like:

- type: linear
    regex: "CRE-2025-0026"
    linear:
      team_id: "9cfb482a-81e3-4154-b5b9-2c805e70a02d"
      secret_env: LINEAR_TOKEN
      title_template: |
        [{{ field .cre "Id" }}] {{ field .cre "Title" }}
      description_template: |
        {{ stripdash (field .cre "Description") }}
        ### Impact
        {{ stripdash (field .cre "Impact") }}
        ### Cause
        {{ stripdash (field .cre "Cause") }}
        ## Mitigation
        {{ field .cre "Mitigation" }}
        {{- $refs := field .cre "References" -}}
        {{- if $refs }}
        ### References
        {{ range $refs }}
        - {{ . }}
        {{ end }}
        {{- end }}
        +++ ## Events
        {{- range .hits }}
          ```
          {{ .Entry }}
          ```
        {{- end }}
        +++

Also some CRE fields lead with a dash character which causes them to render as list items when they're interrupted as Markdown. So I added a stripdash function to the template helpers.

[tonymeehan]

Will merge once the CLA is signed! 🎸