Remove MCP backend, entrypoint injection, and docs; add migration shim

Completes the removal begun in the previous commit.

Backend: deletes models/mcp_server.rs, storage/mcp_store.rs and
commands/mcp_commands.rs, the McpStore on AppState, the four IPC
handlers, Project::enabled_mcp_servers, build_mcp_servers_json(),
compute_mcp_fingerprint(), the MCP_SERVERS_JSON env injection, the
mcp-fingerprint label, and the whole MCP container lifecycle.
create_container() and container_needs_recreation() lose their
mcp_servers/network_name parameters.

Container: entrypoint.sh no longer merges MCP_SERVERS_JSON into
~/.claude.json. MCP_SERVERS_JSON stays in the reserved env blocklist.

Security: the Docker socket is no longer auto-mounted for stdio+Docker
MCP servers — it now mounts only when allow_docker_access is set.

Migration: old containers were created with
network_mode=triple-c-net-<projectId> and refuse to start once that
network is gone. docker/network.rs becomes docker/legacy_cleanup.rs with
label-driven, best-effort removal of leftover MCP containers and the
per-project network, called on both delete and recreate.
container_needs_recreation() now forces a rebuild for any container
carrying a non-empty triple-c.mcp-fingerprint label or attached to a
triple-c-net-* network, moving it onto the default bridge. Both can be
dropped a release later.

Docs: drops the MCP sections from README/HOW-TO-USE/TECHNICAL and adds a
short note pointing at Claude Code's native `claude mcp` / `/mcp` /
.mcp.json instead.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

app/src-tauri/src/commands/file_commands.rs

@@ -157,9 +157,10 @@ pub async fn download_container_file(
 ///   - the workspace (default /workspace), minus regenerable build artifacts
 ///     (node_modules, target), under `workspace/`, and
 ///   - a sanitized copy of the home config under `home-claude/`: ~/.claude.json
-///     with secret-bearing keys removed (mcpServers/settings kept) and ~/.claude/
-///     minus the OAuth `.credentials.json`, so MCP servers, settings and skills
-///     set up via Claude Code survive a Reset.
+///     with secret-bearing keys removed (`mcpServers` — Claude Code's own native
+///     MCP config — and `settings` are kept) and ~/.claude/ minus the OAuth
+///     `.credentials.json`, so settings and skills set up via Claude Code
+///     survive a Reset.
 /// `.git` is kept in full so the backup faithfully preserves git history,
 /// including unpushed commits. Build + gzip happen inside the container so a
 /// large workspace isn't streamed in full. The container must be RUNNING (the