docs: say what these things are, not where they used to be

mara on #3923: "past tense is itself a smell for docs, we dont need any
'used to be somewhere else'" and "any gate we add should not have false
positives". Those are one instruction: the entire false-positive set of a
repo-path checker was prose naming files that are deliberately gone, so
removing the prose is what lets the gate be strict instead of carrying a
skip-list nobody maintains.

Three sites, and only two of them are past tense:

`agent-hierarchy.md` named two removed modules to explain that a code path
is gone. A reader cannot act on where it used to live; they can act on
where cancellation happens now, which is the half the sentence buried.

`term_msg.rs` carried a "Design history" paragraph whose live content was
three rules -- classification is server-side, `level` carries styling, four
named fields are deliberately absent -- wrapped in narration about a
redesign and a pointer to a deleted frontend file. Restated as the rules.

`gotchas.md` is NOT past tense: it correctly names a file in the website
repo. Qualifying it as `hyperhive/website:nix/options.nix` did not work --
the extractor still matches `nix/options.nix` as a substring, so the fix
has to be prose that contains no repo-relative path at all. Naming the
file alone does that and reads better.

Verified against this tree rather than the default checkout: the audit
script takes a directory argument and defaults elsewhere, so its first
(unchanged) DEAD:3 was a true statement about a different branch. On this
one: DEAD 3 -> 0, 141 distinct paths enumerated, and an injected dead path
is still caught.

Refs #3923

docs/process/gotchas.md

@@ -472,7 +472,7 @@ Pipeline:
 - CommonMark from `nixosOptionsDoc.optionsCommonMark` is the only
   output — the source of truth, emitted as `.md`.
 - **HTML + CSS is rendered downstream by the website repo**
-  (`nix/options.nix` there), which consumes this bundle's `host.md` /
+  (its own `options.nix`), which consumes this bundle's `host.md` /
   `agent.md`, renders them with `cmark-gfm`, and shares one
   stylesheet (`docs.css`) across `/options/` and the prose `/docs/`
   tree. Keeping rendering in the website means the theme has a single