feat: expose changed files as {{.Files}} in webhook prompt templates

[knechtionscoding]

What type of PR is this?

/kind feature

What this PR does / why we need it:

Kelos already fetches a pull request's changed-file list for filePatterns filtering (fetchPRChangedFilesWithToken in internal/webhook/github_files.go) and then discards it once matching is done. This PR exposes that list to prompt templates as {{.Files}} via ExtractGitHubWorkItem, so spawners can pass the set of touched files into the task prompt without an extra runtime gh call.

Details:

• Files is always present in the template var map so {{.Files}} never trips the renderer's missingkey=error.
• For PR events it is only populated when a matching filter's filePatterns forced the changed-files fetch (fetching is gated on filePatterns); otherwise it renders as an empty list.
• Push events populate {{.Files}} from the payload regardless.
• Iterate with {{range .Files}}.

This resolves the "deferred to a follow-up" NOTE previously left on the ChangedFiles field. Scope is intentionally limited to .Files only — .Diff would require fetching per-file patch content (larger payloads, truncation concerns) and is left as a future change, consistent with keeping API surfaces minimal.

Which issue(s) this PR is related to:

N/A

Special notes for your reviewer:

{{.Files}} for PR events depends on filePatterns being set on the matching filter, because that is the only thing that triggers the lazy fetch. This coupling is documented in docs/reference.md. Adding a .Diff variable or fetching files when the prompt references {{.Files}} without filePatterns are possible follow-ups.

Does this PR introduce a user-facing change?

TaskSpawner/SessionSpawner GitHub webhook prompt templates can now reference `{{.Files}}`, the list of changed file paths. It is populated for push events, and for pull request events when the matching filter declares `filePatterns`.

---

Summary by cubic

Expose changed file paths to webhook prompt templates as {{.Files}} so tasks can include touched files without extra GitHub calls. For PRs, it is populated only when a matching filter’s filePatterns triggered a fetch; push events always provide it from the payload.

• New Features
  • Add {{.Files}} (list of paths). Always present; may be empty. Iterate with {{range .Files}}.
  • Update docs/reference.md and add tests for rendering and empty behavior.

^{Written for commit eeb26b7. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 4 files

_{Re-trigger cubic}

[gjkim42]

Looks good, one nit for naming.

[gjkim42]

Is it better to name it ChangedFiles?