Add a manual live-server smoke test suite

从 docs/confluence/ 以及现有的 golden 和 --json 测试覆盖开始，了解哪些实时 Confluence 行为尚未经过测试。围绕页面宽度、附件同步、可接受的附件名称和已发布表格的渲染，定义最小的手动调用测试套件。完成的标准是：明确规定调用方式、凭据和个人空间的处理、保护措施、清理、断言，以及将其排除在 go test ./... 和 CI 之外。

Add a test suite a developer runs by hand against a real Confluence server —
credentials from the usual CONFLUENCE_* resolution, pages created in a personal
space — covering a small, critical set of behaviour.

Why

The current suite proves what markfluence emits: the converter's goldens pin the
storage format, and the --json schema guards the output contract. Neither can
prove what Confluence does with what we send, and docs/confluence/
is explicit that this is where confident wrong conclusions come from — body.storage
shows only what was stored, never what takes effect. Several behaviours were
established by experiment and are currently protected by nothing but those notes:

• page width living in two content properties, where writing one leaves the reader
  and the editor disagreeing;
• attachment sync's checksum/recorded-path logic (skip, update, repair a mangled
  path) and the multipart charset requirement;
• what attachment names Confluence accepts;
• whether a published table keeps its layout, alignment, and highlight colours.

A suite that runs against a live server would turn those from documented
observations into something re-checkable when Atlassian changes behaviour.

Deliberately undecided

Nothing about the design is settled yet — to be worked out when this is picked up:

• what it covers (which commands, which round trips, how minimal is minimal)
• how it is invoked and how it is kept out of go test ./... and CI
• how the target space and credentials are supplied
• what guardrails and cleanup it needs, given it writes real pages
• where the files live and what they assert against

Notes

Came up while fixing #18. Not urgent — parked for now.