docs: conclusion job token reference omits SC_WRITE_TOKEN

Salvaged from #1690 (closed as conflicting/superseded), where this hunk was verified accurate.

`docs/conclusion.md` currently says the Conclusion job files/comments on work items using `SYSTEM_ACCESSTOKEN`. That is incomplete: `token_source_for()` in `src/compile/ado_bundle.rs` selects `SC_WRITE_TOKEN` when a write service connection is configured, and the Conclusion job is an explicit caller of that helper (shared with the Stage 3 executor so the two cannot disagree).

Suggested fix — in the `docs/conclusion.md` paragraph describing what the job does:

> ... and then files or comments on Azure DevOps work items using `SYSTEM_ACCESSTOKEN` (or `SC_WRITE_TOKEN` when a write service connection is configured).

Found during a triage sweep of open automated PRs.

Start with the Conclusion job paragraph in docs/conclusion.md, then read token_source_for() in src/compile/ado_bundle.rs to confirm the documented token selection. Done means the reference covers both SYSTEM_ACCESSTOKEN and SC_WRITE_TOKEN when a write service connection is configured, without changing the implementation.

Written by the indexing model from the issue text.