link/claude: secrets in MCP specs, and a separate spec for Desktop

Two things a spec in ~/.config/claude/mcp-servers.d/ may now carry beyond
what Claude Code takes, both resolved at link time:

- "${szecret:NAME}" in any string value, replaced with `szecret get NAME`
  when the server is registered with Claude Code. For a remote server's
  Authorization header: Claude Code's own headersHelper is not run in a
  workspace without persisted trust, which includes $HOME, so a token
  fetched that way is simply missing. A secret that cannot be read skips
  that server, loudly.

- "desktop", a whole spec given to Claude Desktop instead. Desktop only
  starts local stdio servers, so a layer that moves a server to HTTP for
  Claude Code can keep Desktop on a local copy.

dotfiles

Shell, editor and terminal configuration, the tools that go with them, and
szecret, an age-based secret manager whose store lives elsewhere.

History restarts here. What came before was not written to be read by
anyone else, and carried machine names, addresses and an encrypted secret
store; it is kept privately instead. What is left is meant to be safe on
any machine, including one whose owner is not me: see CLAUDE.md for the
rule it is written to.