docs(env): document OMNIROUTE_CORPUS_CACHE_SIZE so Docs Gates goes green again - #11576

The gate is red for everyone

Docs Gates (fast-path) fails on every PR right now, whatever the PR changes.
From my own run on release/v3.8.51
(job):

Env var contract sync report
============================
Code references:          594 unique vars
In .env.example:          783 unique vars
In docs/reference/ENVIRONMENT.md: 801 unique vars

  ✗ In code but missing from .env.example: 1
     - OMNIROUTE_CORPUS_CACHE_SIZE
  ✓ In .env.example but missing from ENVIRONMENT.md: none
  ✓ In ENVIRONMENT.md but missing from .env.example: none

✗ Env / docs contract is out of sync.

check:env-doc-sync is the third link in the check:docs-all chain, so it takes the
whole Docs Gates job down with it.

Cause

#11491
(721ee2a03, today) put an LRU bound on the local-corpus index cache:

// src/lib/localCorpus/configured.ts:41
const maxCacheSize = Math.max(1, parseInt(process.env.OMNIROUTE_CORPUS_CACHE_SIZE || "5", 10) || 5);

The variable is real and works; it just never reached .env.example or
docs/reference/ENVIRONMENT.md, and the contract checker requires both.

The fix

Documents it in both files, in the Memory Optimization section next to the other
cache-size knobs (PROMPT_CACHE_MAX_SIZE, SEMANTIC_CACHE_MAX_SIZE, …), which is where
a reader tuning a low-RAM box would look for it. Behaviour documented from the code:
default 5, LRU eviction of the least-recently-used root, clamped to a minimum of 1,
non-numeric falls back to the default.

No allowlist entry — the variable is a legitimate operator knob, so it belongs in the
contract rather than in the checker's exception list.

Verification

The checker's own parsers, run against the two files after the change:

env.example parsed: 784          (CI reported 783)
ENVIRONMENT.md parsed: 802       (CI reported 801)
OMNIROUTE_CORPUS_CACHE_SIZE in .env.example  -> true
OMNIROUTE_CORPUS_CACHE_SIZE in ENVIRONMENT.md -> true

Both counts land exactly one above what the failing job reported, and the only name in
the failure list is now present in both sets — so the three runEnvDocSync diffs are
all empty.

npx prettier --check docs/reference/ENVIRONMENT.md   # All matched files use Prettier code style!

Docs-only; no code, no tests.