feat: probe stdio MCP servers with an initialize handshake

[feiler0]

What

Extends connectivity probing to stdio MCP servers. Previously mcp_probe, the panel probe action, /mcp <server> probe, and the optional passive probe only supported streamable-http endpoints and rejected stdio rows outright (stdio servers have no HTTP endpoint to probe).

Now a probe on a stdio row:

• spawns the configured command / args with the scrubbed parent env + configured env (same scrubbedParentEnv policy as the mcp-client bridge) and optional cwd,
• completes one MCP initialize handshake over stdin/stdout,
• records the sanitized server name/version or a sanitized failure detail in the panel — still panel-only, with the same job semantics (unowned background job, cancel, per-probe timeout, display cap).

Changes

• src/probe.ts — new ProbeTarget union (http | stdio), probeStdio(), probeJob() dispatch on transport; mcp_probe tool copy updated for both transports.
• src/service.ts — new probeSpec() that resolves either the raw HTTP endpoint or the stdio launch spec per mcp-client row; probe(), the passive sweep, and tool targeting all share it.
• tests/probe.spec.ts — stdio handshake + child-exit cases (real spawned node child, no network).
• CHANGELOG.md — [Unreleased] entry.

Notes

• The published bundle (lib/) is generated by tsdown and is not tracked in this repository, so no build artifacts are included.
• The stdio handshake path was manually validated against real stdio MCP servers before submission (initialize over stdio returns serverInfo).
• Follows the existing read-only / panel-only contract: probe credentials and details never reach logs, snapshots beyond the panel, or model context.