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

docs/ARCHITECTURE.md

@@ -148,7 +148,7 @@ Orchestration logic that commands reference. Contains the step-by-step process i
 Workflow files are loaded verbatim into Claude's context every time the
 corresponding `/gsd-*` command is invoked. The workflow size budget enforced by
 `tests/workflow-size-budget.test.cjs` keeps each file bounded, mirroring the
-agent budget from #2361. The budget is measured in **bytes** (#717), not lines:
+the agent size-budget convention. The budget is measured in **bytes** (#717), not lines:
 line count over-penalizes prose and under-catches token-dense tables and code
 blocks, whereas bytes are deterministic and match the unit our vendors bound on
 — Codex truncates instruction docs past 32,768 bytes (`project_doc_max_bytes`).
@@ -180,7 +180,7 @@ that is still eagerly `@`-imported shrinks the measured file without shrinking
 loaded context, which games the proxy rather than serving the goal.
 
 `workflows/discuss-phase.md` is held to a stricter <30,000-byte ceiling per
-issue #2551 (originally <500 lines; re-based to bytes for #717). When a workflow grows
+the discuss-phase byte budget (#717; the discuss-phase/modes split keeps it ≈32000 bytes). When a workflow grows
 beyond its tier, extract per-mode bodies into
 `workflows/<workflow>/modes/<mode>.md`, templates into
 `workflows/<workflow>/templates/`, and shared knowledge into