Use ProcessInfo.hostName for postd SSE log line

[odrobnik]

What

postd's HTTP+SSE startup log built its URL with String.localHostname:

daemonLogger.info("MCP HTTP+SSE transport listening on http://\(String.localHostname):\(httpTransport.port)/sse")

SwiftMCP 1.5 (pulled in by #25) removed that helper. A Codex review on #25 flagged this as a P1 "won't compile" — but it was a false positive: the daemon kept building and CI stayed green, because SwiftMail 1.6.4 still vends an identical String.localHostname that leaks into PostDaemon.swift through PostServer's transitive import SwiftMail. CI compiles every target (swift build, debug + release, macOS + Linux), so a real break there would have gone red.

So nothing was broken — but the green build leaned on two fragile things:

1. SwiftMail keeping localHostname. SwiftCross's own commit dropping it called it an "API smell inherited from SwiftMail/SwiftMCP," replacing it with the built-in ProcessInfo.hostName plus a new ProcessInfo.localIPAddress. SwiftMail's copy is a likely future removal.
2. Leaky extension-member visibility through a transitive import — which SE-0444 (MemberImportVisibility) would eventually turn into an error requiring a direct import SwiftMail.

Change

Use Foundation's ProcessInfo.processInfo.hostName directly:

• behavior-preserving — the old helper returned ProcessInfo.processInfo.hostName first (IP/localhost only as fallbacks);
• no new dependency (Foundation is already imported; does not pull in SwiftCross, which intentionally has no hostname shim);
• matches SwiftMCP's own convention — HTTPSSETransport.init defaults its host to ProcessInfo.processInfo.hostName.

Verification

• swift build ✅ — post + postd link cleanly against the pinned SwiftMCP 1.5.1 / SwiftMail 1.6.4.

🤖 Generated with Claude Code