docs: format reference links in plan to satisfy rumdl and prettier

The reference link definitions at the end of PLAN.md included multiline
titles and query strings, which caused Prettier to wrap them across multiple
lines without angle brackets and rumdl (MD034) to flag the indented URLs and
re-add angle brackets. This created an alternating conflict on every `just fmt`
invocation.

Simplify the link reference definitions to single-line URLs without query
strings and titles so both Prettier and rumdl format cleanly and idempotently.

Test Plan:
- `just fmt` -- formatted cleanly with no changes across repeated runs
- `just clippy` -- passed
- `just test` -- all 118 unit tests passed

PLAN.md

@@ -615,21 +615,11 @@ each remote player a unique virtual MAC on the real LAN, filters dangerous L2
 control traffic, and keeps the physical LAN gateway as the only machine touching
 the real LAN.
 
-[1]:
-  <https://github.com/OpenVPN/tap-windows6?utm_source=chatgpt.com>
-  "OpenVPN/tap-windows6: Windows TAP driver (NDIS 6)"
-[2]:
-  <https://man7.org/linux/man-pages/man7/packet.7.html?utm_source=chatgpt.com>
-  "packet(7) - Linux manual page"
-[3]:
-  <https://man7.org/linux/man-pages/man7/capabilities.7.html?utm_source=chatgpt.com>
-  "capabilities(7) - Linux manual page"
-[4]:
-  <https://datatracker.ietf.org/doc/html/rfc9221?utm_source=chatgpt.com>
-  "RFC 9221 - An Unreliable Datagram Extension to QUIC"
-[5]:
-  <https://docs.rs/quinn/latest/quinn/struct.Connection.html?utm_source=chatgpt.com>
-  "Connection in quinn - Rust"
+[1]: https://github.com/OpenVPN/tap-windows6
+[2]: https://man7.org/linux/man-pages/man7/packet.7.html
+[3]: https://man7.org/linux/man-pages/man7/capabilities.7.html
+[4]: https://datatracker.ietf.org/doc/html/rfc9221
+[5]: https://docs.rs/quinn/latest/quinn/struct.Connection.html
 
 I want a mono-repo, Rust code, crates into a "crates" folder, one cargo
 workspace.