feat(agent): add MCP toolchain shims with config isolation

[wpfleger96]

Summary

Bundled shell shims that resolve bare uv, uvx, npx, and node commands to hermit-managed, config-isolated toolchains for MCP server spawning. This fixes the incident where a salt-managed /etc/uv/uv.toml with unsupported system-certs broke all MCP servers using bare uv commands.

Shell shims (crates/buzz-agent/resources/shims/)

• setup-common.sh: shared hermit bootstrap with SHA-verified installer, mkdir-based lock with PID+timestamp stale detection (600s), isolated HERMIT_STATE_DIR, macOS PATH fix, Linux temp-binary workaround. Logging to $BUZZ_MCP_LOG_DIR/mcp-shim.log with umask 077 — never logs arguments or env var values.
• uv/uvx: UV_NO_CONFIG=1 suppresses all uv config file discovery — direct defense against the incident.
• npx/node: two distinct empty npmrc files (empty-user-npmrc + empty-global-npmrc) avoid npm's double-load crash, NPM_CONFIG_LOCATION=global suppresses project .npmrc discovery.

Rust changes

• Shim resolver (mcp.rs): pure cfg(unix) resolve_shim_command(command, shim_dir) rewrites bare uv/uvx/npx/node to <shim_dir>/<command> when the shim file exists. Absolute paths and unknown commands pass through unchanged.
• Spawn-boundary env construction (mcp.rs): build_child_env() / build_child_env_with() propagates BUZZ_MCP_SHIM_DIR, BUZZ_MCP_HERMIT_DIR, and BUZZ_MCP_LOG_DIR through env_clear() into the child process, then applies spec.env with last-write-wins semantics so explicit server overrides take precedence.
• Config timeout (config.rs): mcp_init_timeout default bumped from 30s to 300s (matches Goose). Env-overridable via BUZZ_AGENT_MCP_INIT_TIMEOUT_SECS.

Desktop integration (runtime.rs, tauri.conf.json)

• Shims bundled as Tauri resources with ensure_shims_executable() for execute-bit preservation.
• BUZZ_MCP_SHIM_DIR set from resource_dir/shims (read-only bundle resources).
• BUZZ_MCP_HERMIT_DIR set from app_data_dir/mcp-hermit (writable state) — separated from the signed app bundle.
• BUZZ_MCP_LOG_DIR set from data_local_dir/xyz.block.buzz.app/logs.

Tests

Unit tests (9 new in mcp.rs):

• 8 shim resolver tests: bare command resolution for all 4 commands, absolute path passthrough, unknown command passthrough, None shim dir, missing shim file.
• 1 spawn-boundary test (build_child_env_propagates_shim_vars_and_spec_overrides): verifies all three BUZZ_MCP_* variables propagate through build_child_env_with() and that explicit spec.env overrides win via last-write-wins.

Integration tests (7 in tests/shim_hostile_config.rs):

• Shim file existence + execute bits for all 5 checked-in scripts.
• npm_uses_distinct_config_files: user and global npmrc are separate files.
• Real wrapper suite (5 tests): invokes the actual checked-in uv, uvx, npx, and node wrappers through setup-common.sh with a pre-seeded fake hermit dir containing probe scripts. Verifies UV_NO_CONFIG=1 for uv/uvx, npm isolation vars for npx/node, and explicit UV_INDEX_URL override passthrough. No network dependency.