feat(go): allowlist launch channel srcs

[ErikBjare]

Adds newsletter, producthunt, hackernews, reddit, x, linkedin and discord to the /go/ SRC_DEFAULT allowlist (all → subscribe), so Pro links in the v0.14.0 launch posts are attributed per channel in GA4 instead of falling into other. Same pattern as #80 (release-notes) and #84 (blog).

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[ErikBjare]

@greptileai review

[greptile-apps]

Confidence Score: 5/5

[Medium risk] Adds launch channel tracking to a redirect router.

This PR appears safe to merge.

What we checked:

• Launch labels stay intact: KNOWN_SRC comes directly from SRC_DEFAULT. All seven labels pass the existing cleanup unchanged, and GA4 receives the accepted src.
• Explicit destinations still win: Each new label maps to the existing subscribe URL. An allowed to value is checked before the source default.

Summary

Adds seven launch channels to /go/: newsletter, producthunt, hackernews, reddit, x, linkedin, and discord.

• Each channel defaults to subscribe and keeps its own label in the GA4 nudge_click event.
• Explicit destinations and the existing redirect safeguards remain unchanged.
• No actionable issues found.

_{Reviews (1) · Last reviewed commit: "feat(go): allowlist launch channel srcs ..."}