v0.45.0 — mergeability, comments, run sha; Azure board check

Three additions to the contract an agent reads, and an Azure board check that now happens before anything is written.

Added

• Why a pull request can or cannot merge (#232). scm.pr.status / baron_scm_read op=pr_status carries mergeability: mergeable, conflicts, computing, blocked_by_policy, failed or unknown. Azure used to collapse its trial-merge status to a boolean, so queued, a policy refusal and a failure all looked the same. GitHub maps its mergeable_state. A PR that is no longer open reads unknown, so Azure's lingering "succeeded" is never mistaken for "merged". The boolean mergeable stays for existing callers.
• Reading an item's comments (#237). baron_issue_read op=comments and the issue.comments recipe op return every comment, oldest first, on Azure DevOps, GitHub, Jira and Linear. Read before posting so the same evidence is not added twice.
• The commit a CI run built (#237). Runs carry sha (Azure sourceVersion, GitHub head_sha), so a run, and the deploy after it, can be tied to a merge commit.

Fixed

• Azure: a Kanban column the board does not have (#239). A backlog item asked for a column its board lacks used to read as reachable, and only Azure's refusal of the write caught it, past the point where gapPolicy.separateBoardColumn could decide. The board is now read first: a missing column, or one whose state mapping disagrees with the target state, is reported before anything is written, and the board's own spelling of the column is what gets written.