feat(optimize): add --subtract, the subtraction axis, behind a deterministic floor

Every command so far asked an addition question — what to add, what to move,
what it costs. Nothing asked what is no longer earning its always-loaded rent.
This adds that axis as a fourth lensCheck on the existing hybrid motor rather
than a new scanner or a 22nd command: the measured payoff (~18% of one file)
justifies a mode, not machinery.

It is the only lens that proposes REMOVING config, so it carries a guarantee
the others don't need: a load-bearing block is never a candidate. Precision is
asymmetric — a missed dead line costs a few tokens per turn, a deleted one
costs a wrong remote or a broken script — so the floor is decided in code
(lib/floor-exclusion.mjs) before the opus judge sees anything, never in prose.

Granularity is the leaf block, with two structural exceptions: a paragraph
ending in ':' merges with the list it introduces, and an ordered list is a
contract whose steps inherit floor from any sibling. Unordered lists
deliberately do not inherit — a load-bearing bullet and a disposable one
routinely share a list, and container-reasoning is the error the hand-built
ground truth exists to catch.

Verified against that ground truth (built before any classifier existed), with
the comparison machine-checked rather than read by eye: zero load-bearing
blocks proposed, 11/18 deletable groups surfaced, ~756 tok ~ 18% of a ~4300
token file — inside the pre-registered band. The first run found five floor
violations the synthesized fixture missed; each got a structural rule and a
fixture shape so it cannot regress.

Three real bugs the dogfood run exposed, all now covered:
- JS \b is ASCII-only, so /\bunngå\b/ never matches — every Norwegian keyword
  ending in æ/ø/å was silently dead.
- A bare word/word is not a path; "pros/cons" vetoed the largest deletable
  block until PATH_RE was tightened to rooted paths and globs.
- "Mid-sentence" must key on a preceding lowercase letter; the loose version
  read **bold labels:** and quoted openers as entities, costing 4 of 11 groups.

BP-SUB-001 is grounded entirely in the Anthropic steering blog already cited by
BP-MECH-001..004 and asserts nothing from the talk that motivated the feature —
no "80%", no ablation figure.

Suite 1365 -> 1382/0. Frozen v5.0.0 snapshots untouched; plain optimize output
byte-identical on identical input (--subtract adds keys only when passed).
knowledge-refresh-cli's reference date moved to 2026-08-01: its premise that
every seed entry was verified 2026-06-20 expired when BP-SUB-001 got a genuine
verification date, and backdating the entry to fit the test would have been a
lie about when its source was checked.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RW2haJXbxZpKivKHseSXNh

knowledge/best-practices.json

@@ -136,6 +136,18 @@
       "category": "size-budget",
       "lensCheck": "CA-SKL-002",
       "source": { "url": "https://code.claude.com/docs/en/skills", "title": "Skills", "verified": "2026-06-20" }
+    },
+    {
+      "id": "BP-SUB-001",
+      "claim": "Every line of CLAUDE.md loads into every session whether or not it is relevant, which consumes tokens and dilutes adherence. A line that states a local fact the model cannot derive (build commands, directory layout, conventions, team norms) earns that cost; a line that only restates general engineering behaviour pays it without being the kind of content CLAUDE.md is for, and is a candidate for removal.",
+      "mechanism": "deletion",
+      "appliesTo": "claude-md",
+      "recommendation": "Review the block for removal, then re-add it only if the model actually stumbles on it repeatedly. Local facts (remotes, versions, paths, conventions) and policy invariants are the floor and are never removal candidates.",
+      "confidence": "confirmed",
+      "severity": "low",
+      "category": "subtraction",
+      "lensCheck": "compensatory-instruction",
+      "source": { "url": "https://claude.com/blog/steering-claude-code-skills-hooks-rules-subagents-and-more", "title": "Steering Claude Code: skills, hooks, rules, subagents and more", "verified": "2026-07-31" }
     }
   ]
 }