The sensor whose every finding is "delete it"

## Spark

Two weeks on partir.qua.world spent deleting — a scaffold module `cabal init` left, two accessors nothing called, comments that read the code back — then wiring weeder into CI so the deletions stay done. A Readwise highlight from the same window named the principle the whole arc was circling.

## Why it could be interesting

Most computational sensors tell you to write something: a missing test, an unhandled case, a type that doesn't line up. A reachability gate is the one whose output is always subtraction, which makes it the cheapest sensor to act on and the one most likely to be argued with, because every finding is code someone chose to write. The claim: a reachability gate is worth more on a young repo than a mature one, because it catches scaffolding before scaffolding acquires defenders — and the highlight's inversion of the single responsibility principle ("each hard problem is only handled by one module") is the test for which of the two duplicate paths to delete.

## Open questions

- [ ] Did weeder find anything that wasn't scaffolding or an unused accessor? The post needs at least one finding that was genuinely surprising, or it's a tool announcement.
- [ ] Does the "each hard problem handled by one module" framing actually decide any of this window's deletions, or is it a nicer-sounding restatement?
- [ ] Relationship to #189 (computational vs inferential sensors) and #421 (every convention gets an owning sensor) — is this a worked example of those, or its own claim?
- [ ] Whether agent-written code changes the calculus: #652 argues size and complexity rules matter more when an agent wrote the code. Same argument for reachability?

## Source material

- qua-world/partir.qua.world#59 → PR https://github.com/qua-world/partir.qua.world/pull/98 ("Fail CI on code nothing reaches")
- Commits: "Delete the scaffold module cabal init left", "Drop the two accessors nothing calls", "Cut the weeder comments back to what earns a line", "Cut the documentation back to what the code cannot say"
- Readwise highlight (2026-09-01), *Write Code That Is Easy to Delete, Not Easy to Extend*: https://read.readwise.io/read/01m1f055cv1796j2m8apf7436z
- qua-world/partir.qua.world#89 (data clumps and unchecked `fromJust`)
- #652, #189, #421