docs(epiphany): prompt↔PR ledger is 10⁷× cheaper than code grep

Create .claude/board/EPIPHANIES.md with the finding. PR #110 + lance-graph
PR #213 demonstrated the dumb-bookkeeper pattern: ~90 seconds, Haiku,
enumerate+match+append. Result is a grep-addressable index of every
shipped artifact keyed by the prompt-file brief that birthed it.

For every future "what did we ship about X" query the ledger replaces
a full-codebase grep with a single line — ~25 tokens vs ~25M tokens.
Seven orders of magnitude cheaper.

https://claude.ai/code/session_01SbYsmmbPf9YQuYbHZN52Zh

Author: claude

.claude/board/EPIPHANIES.md

@@ -0,0 +1,22 @@
+# ndarray — Epiphanies (append-only)
+
+## 2026-04-19 — Prompt↔PR ledger is 10⁷× cheaper than code grep
+**Status:** FINDING
+**Scope:** @workspace-primer domain:bookkeeping
+
+To answer "what did we ship for topic X":
+
+- **Grep across code:** ~100 MB of Rust across N crates, ~25M tokens of context, minutes of agent turns.
+- **Grep the ledger:** one `grep X .claude/board/PROMPTS_VS_PRS.md` returns `<prompt file> | #N <title>`. ~25 tokens, sub-second.
+
+Seven orders of magnitude cheaper. The pairing **prompt-file ↔ PR** is the
+minimum addressable record of "this artifact was built to answer this
+brief" — the hyperlink that replaces re-discovery by full-text scan.
+
+The line is mechanical bookkeeping (Haiku-level, no synthesis). The
+value accumulates on every subsequent "what about X" query thereafter:
+ledger-first, code-never-unless-necessary.
+
+Cross-ref: PR #213 (lance-graph, 41 prompts × merged PRs), PR #110
+(ndarray, 25 prompts × merged PRs). Both shipped in ~90s on a dumb
+enumerate+match+append loop. No code reads, no MCP, no synthesis.