fix(session): page v1 message history with a before cursor

[e-n-0]

What

GET /session/{id}/message now accepts before=<messageID> and returns an x-next-cursor header (the oldest message ID in the page) while older messages remain.

Why

OpenChamber loads history with limit=50 and only asks for older pages when it gets an x-next-cursor back. Antechamber never sent one, so the UI took the newest 50 messages as the whole history. In a long session where those 50 are all assistant replies from one turn, there is no user message to group them under, and the session opens blank.

How to verify

npm run typecheck     # passes
npx tsx --test --import ./test/setup.ts test/protocol-v124-sessions-history.test.ts   # 3/3 pass
npm run build         # succeeds
npm run format:check  # clean

V124-006 now also checks that the cursor header is set, that before returns the older page, and that the last page has no cursor. I also checked a real 396-message OmO session by hand: paging with limit=50 returned all of it in 8 pages. No smoke script was run.

Checklist

• npm run typecheck passes
• npm test passes (session-history file)
• npm run build succeeds
• New/changed behavior has a test; no fixed sleeps were added
• Commit messages follow Conventional Commits
• docs/api-surface.md updated if routes/events changed
• docs/configuration.md updated if an environment variable was added, renamed, or had its default changed (n/a)
• No personal paths, hostnames, or credentials in the diff

[e-n-0]

already merged on main: close #13