Format-preserving writer for [mcp.servers] entries in agent config #402

Summary

The MCP install helper (#310) needs to add and update [mcp.servers.<name>] entries in a user's config.toml "without destroying unrelated settings" (epic acceptance criteria). Today the agent config is read-only: aura-config only parses, and the format-preserving toml_edit machinery in aura-cli/src/config.rs is scoped to the CLI's own cli.toml.

This issue adds the reusable building block: a writer in aura-config that upserts a single MCP server entry into an existing config file.

Scope

• aura_config::writer::upsert_mcp_server(path, name, &McpServerConfig) plus a string-level variant for testability.
• Format-preserving via toml_edit: unrelated tables, sibling servers, comments, and value decor survive the edit.
• Atomic write (temp file + rename in the same directory), matching the existing cli.toml save path.
• {{ env.VAR }} placeholder strings pass through verbatim, so callers can keep secrets out of the TOML.
• Creates [mcp] / [mcp.servers] as explicit tables when missing; replaces an existing server table wholesale on name collision.
• Errors (rather than clobbering) on malformed TOML or a missing config file.
• Written output must round-trip through the normal loader back into an equal McpServerConfig for all three transports (stdio, http_streamable, sse).

Non-goals

• No user-facing command (the guided /mcp flow builds on this separately).
• No remove/rename/list operations.
• No change to worker mcp_filter semantics ([FEATURE]: Empty worker mcp_filter grants access to all MCP tools #378).