Open the shared board's session for clients that never initialize - #23

Fixes #24.

Clients on MCP 2026-07-28 never send initialize. They send server/discover, then name
themselves in every request's _meta. Claude Code 2.1.309 is one such client. rmcp 3.8.0
serves these clients fine on a private board, but on a joined board every tool call returns
The shared board answered 400.

A joined kanzo mcp opens its session on the host by forwarding the client's own
initialize (peer_info()). These clients never sent one, so Kanzo forwards
"params": null. The host answers 400 Bad Request: Session ID is required, and answers the
same to notifications/initialized and to the tools/call.

This change builds that initialize from the request's _meta when the client never sent
one:

• It uses 2025-11-25, rmcp's LATEST_WITH_INITIALIZE, since 2026-07-28 itself has no
  initialize.
• It names the client agent when the request gives no name.
• A client that did send initialize is forwarded exactly as before.

add_comment takes its default author the same way: from _meta on a 2026-07-28 request,
and from initialize otherwise, as before. Over HTTP, rmcp gives a stateless request a
stand-in initialize named rmcp, so a stateless client that gave no name was credited as
rmcp. It is now credited as agent, as it is over stdio.

A joined board still opens one host session per kanzo mcp process, so that session keeps
the name from the process's first call. Clients send the same name on every call, so this
shows only if a client's name changes partway through.

Three new tests run the real server in process:

• A stateless client's comment, sent through a joined board, reaches the host credited as
  claude-code. On main it gets the 400.
• A client that sends initialize, and a different name in _meta, is still credited with
  its initialize name.
• A stateless HTTP client that gives no name is credited as agent.

Two unit tests pin the forwarded initialize: its version, its name and capabilities taken
from _meta, and a client's own initialize passed on unchanged.

I tested it against a board shared from another machine:

• With Claude Code 2.1.309's own messages replayed, 0.3.0 returns the 400, and this branch
  lists the projects.
• A client that sends initialize still works.
• A live Claude Code session's calls succeed once its kanzo mcp runs this build.

No MCP reply changes size.

• cargo fmt --check, cargo clippy --all-targets -- -D warnings and cargo test pass
• A board change was tried in a browser, in light and dark mode: not applicable, nothing
  on the board changes
• The README says what changed for users, if anything did: nothing changes for users