fix(mcp): support X-Forwarded-Proto in SSE endpoint scheme

[sjhddh]

When the MCP SSE server runs behind a TLS-terminating reverse proxy (nginx, Caddy, ALB), r.TLS is nil so the advertised endpoint URL ends up as http://..., breaking clients that connected over https://.

This adds an X-Forwarded-Proto fallback before defaulting to http, matching standard reverse-proxy conventions. r.URL.Scheme (when explicitly set) still wins.

scheme := "http"
if r.TLS != nil {
    scheme = "https"
} else if proto := r.Header.Get("X-Forwarded-Proto"); proto != "" {
    scheme = proto
}
if r.URL.Scheme != "" {
    scheme = r.URL.Scheme
}

Replaces #65 (closed; that PR could not be reopened because the branch was force-pushed to fix the commit author identity).

[mxnstrexgl]

🤖 Hermes Agent Auto-Review — PR #113

SSE endpoint X-Forwarded-Proto fix

⚠️ Warning

• X-Forwarded-Proto is trust-dependent: This header can be spoofed by clients if the server is directly exposed to the internet without a trusted reverse proxy stripping/re-setting the header. An attacker can set X-Forwarded-Proto: https to trick the SSE endpoint into generating https:// scheme URLs even when behind HTTP. Ensure this code path is only reachable behind a trusted proxy that sets this header.

✅ Looks Good

• Minimal, targeted fix — correctly falls through to the existing r.URL.Scheme check.
• TLS check takes priority — r.TLS != nil still wins, which is correct.
• Fixes real issue — SSE URLs would be wrong behind a TLS-terminating proxy without this.

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Reviewed by Hermes Agent

[mxnstrexgl]

🤖 Hermes Agent Security Review

Verdict: Changes Requested — 1 warning-level issue.

⚠️ Warning

X-Forwarded-Proto header trusted without validation — The value is used directly as scheme without checking it equals "http" or "https". Any client (not just a trusted reverse proxy) can set this to an arbitrary string like javascript:// (potential reflected XSS in URL rendering) or //evil.com (potential redirect hijacking).

Fix (2 lines):

} else if proto := r.Header.Get("X-Forwarded-Proto"); proto == "http" || proto == "https" {
    scheme = proto
}

✅ Clean

• No SSRF risk (scheme used only for response URLs, not outbound requests)

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Reviewed by Hermes Agent (cron auto-review)