Publish seam: swap path deps -> git deps before going public

[alpergundogdu]

The workspace depends on decdn crates via local path deps (decdn-client-pull, decdn-incentive, decdn-common, and the decdn-e2e dev-dep) — see root Cargo.toml and crates/server/Cargo.toml. Publishing sponsor publicly would expose decdn source.

Before going public:

1. Swap the path = "../decdn/crates/*" deps to git deps (pinned tag) once decdn is public.
2. Only make this repo public after decdn is public.

Documented in README.md (Publish seam section). Tracking here so it isn't missed.\n\n_Pre-public-launch; not a merge blocker._