docs(agents): add a verification fidelity ladder

[GenericJam]

What

Six rungs from static analysis to a published artifact, with the two rules that
outrank them: never substitute a lower rung silently, and verify effects rather
than exit codes.

Rungs 4 to 6 are the ones that have cost real time here — the emulator pool that
was uniformly Android 13 and so could not see an API-33 crash on Android 11, the
release path that links plugin NIFs differently from debug, and the debug
defaults that masked a crash until the app was installed from Play.

Why

Borrowed from GenericJam/insider, which
gates agent work on a named fidelity ladder plus one rule: a lower rung may not
stand in for a higher one.

The principle was already here in pieces — "verify effects, not exit codes" in
the agentic guide, the deliberate fast/slow split in .githooks/pre-push — but
there was no named ladder an agent could point at, and therefore no vocabulary
for the sentence that matters: "I verified at rung 3; rung 4 was unavailable
because no device was attached." Today an agent that runs mix test and reports
success is not lying, and that is the problem.

Every rung cites something that actually got past the rung below it, so the list
reads as history rather than process.

Evidence

Docs only — no code paths touched. .githooks/pre-push ran clean on push
(format, Credo --strict with ex_slop, compile --warnings-as-errors).

Not included

Deliberately left for a separate discussion rather than bundled here:

• pre-commit with insider's check-test-pairing co-change gate
• a PR template with an evidence block, and CODEOWNERS over AGENTS.md,
  .githooks/ and decisions/
• insider's commit-msg Conventional Commits hook — not recommended as
  written: its regex rejects 30 of the last 40 subjects on master, including
  every Bump to x.y.z release commit and the MOB-nnn: style