chore(#1073): purge phantom pre-migration issue refs from source, tests, docs (#1471)

#2551/#3182/#2361 are pre-migration get-shit-done-redux issue numbers with no
equivalent in open-gsd/gsd-core; they mislead triage and manufacture phantom
blockers. Repoint to real successors (#717 byte-budget rework, #720) or rewrite
as prose referencing the discuss-phase/modes progressive-disclosure split.
Correct co-located 'line budget'/'<500 lines' framing to the byte-based reality
(#717). Add a CI guard (tests/no-phantom-issue-refs.test.cjs) that fails if a
phantom ref is reintroduced. SSH-key patterns (id_ed25519) left untouched.
No user-facing runtime behavior change.

Closes #1073

CONTRIBUTING.md

@@ -847,7 +847,7 @@ gsd-core/
                           pattern: workflows/<name>/modes/*.md +
                           workflows/<name>/templates/*. Parent dispatches
                           to mode files. See workflows/discuss-phase/ as
-                          the canonical example (#2551). New modes for
+                          the canonical example (the discuss-phase/modes split, #717). New modes for
                           discuss-phase land in
                           workflows/discuss-phase/modes/<mode>.md.
                           Per-file sizes are pinned by a committed baseline