feat: record AI client metadata for telemetry

[hubgan]

Summary

Adds a coarse, anonymous signal to tool telemetry: which AI coding tool is driving the MCP server (Claude Code, Codex, Cursor, Gemini, VS Code, Windsurf, Zed, opencode, Copilot, or other).

How it works

1. Source of truth — the MCP initialize handshake clientInfo.name, read at runtime via Server.getClientVersion(). New ai-identity.ts in @argent/telemetry canonicalizes that raw name to a known slug. Unrecognized tools become other plus a bounded, sanitized free-form name.
2. Cross-process hop — telemetry lives in the tool-server, but clientInfo lives in the MCP server process. The MCP server forwards the identity as X-Argent-AI-Client / X-Argent-AI-Client-Name request headers.
3. Re-validation at the boundary — the tool-server re-checks both headers against the same allowlist/pattern the sanitizer enforces, so a misbehaving client can't inject arbitrary telemetry values. The free-form name is retained only when ai_client === "other".
4. Threading through events — AiTelemetryProps is mixed into the tool event types and merged in the registry listener and tool-server stop handler via the shared aiTelemetryFromMeta helper, so keys are omitted entirely when absent.

Privacy

It records only which tool is connected. It is validated at the mcp-server, re-validated at the tool-server, and enforced a third time by the sanitizer allowlist before anything reaches PostHog. Path-like or oversized names are dropped.