dx/test: mcp add --header-command flag, tui-migrate tests, golden upgrade fixture

Found during v0.9.1 release review (Data-Engineer). Deferred DX/test debt: (1) `altimate mcp add` has no `--header-command` flag (parity with `--header`) — users must hand-edit JSON for bearer-auth. (2) `tui-migrate.ts` (extracts theme/keybinds/tui → tui.json) has no dedicated tests; a silently-failed migration drops customizations without error. (3) No end-to-end golden-fixture upgrade test loading a real 0.8.10 `opencode.json` + `opencode-local.db` and asserting session/config/theme parity on 0.9.1.