docs(agents): add a verification fidelity ladder

[GenericJam]

What

Tuned for a repo whose failures are mostly failures to notice that nothing
happened.

Rung 3 is running the task against a real project, because a task can unit-test
green and still skip its real work. Rung 4 is proving the deployed app answers
rather than trusting the deploy tool. Rung 6 is compiling from the packed
tarball, because Hex omits repository-root dotfiles and two releases have
already shipped broken that way.

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