docs(epiphany): code-arc knowledge loss is 30-50% of session tokens (ambient)

Third finding appended to EPIPHANIES.md — orthogonal to cold-start tax
(20-30 turns) and find-code discount (10⁷×). This is the AMBIENT channel:
30-50% of every session's token budget burns on rediscovering what code
paths exist / what was tried / why code is shaped the way it is.

The prompt↔PR ledger collapses all three channels to two text-file reads:
- Cold-start: 20-30 turns → 3-5 turns   (~6×)
- Find-code:  ~25M tokens → ~25 tokens  (10⁷×)
- Ambient:    30-50% → 0%               (2×-eternal)

https://claude.ai/code/session_01SbYsmmbPf9YQuYbHZN52Zh

Author: claude

.claude/board/EPIPHANIES.md

@@ -20,3 +20,28 @@ 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.
+
+## 2026-04-19 — Code-arc knowledge loss is 30-50% of session tokens (ambient)
+**Status:** FINDING
+**Scope:** @workspace-primer domain:bookkeeping
+
+Empirical (per user, 2026-04-19): **30-50% of session tokens** burn on
+rediscovering what code paths exist, what was tried, what got reverted,
+what decisions led to the current shape. This is **orthogonal** to the
+20-30-turn cold-start tax — it's the *ambient* loss across every query,
+every subagent spawn, every refactor.
+
+The ledger closes three channels at once:
+
+| Channel | Before | After | Discount |
+|---|---|---|---|
+| Cold-start (once per session) | 20-30 turns | 3-5 turns | ~6× |
+| Find-code (per query) | ~25M tokens (grep codebase) | ~25 tokens (grep ledger) | 10⁷× |
+| **Ambient arc knowledge (every turn)** | **30-50% of session budget** | **~0%** | **2×-eternal** |
+
+All three channels collapse to two text-file reads: PROMPTS_VS_PRS.md +
+PR_ARC_INVENTORY.md. The second file is read only when arc detail is
+needed (Knowledge Activation trigger), so the routine cost is 0.
+
+Cross-ref: PR #110 (ndarray ledger), PR #213 (lance-graph ledger).
+EPIPHANIES.md 10⁷× finding above.