fix(cli): stats told operators to set a variable that does not exist - #160

The bug

agentmetry stats answered a disabled audit export with:

Audit export disabled, enable AGENTMETRY_AUDIT_EXPORT to collect stats.

There is no such setting. The only alias is AGENTMETRY_AUDIT_EXPORT_ENABLED,
declared on Settings in core/config.py. AGENTMETRY_AUDIT_EXPORT appeared
exactly once in the entire codebase: inside the string telling somebody to set
it.

The cost is the worst shape this project has. The operator reads the message,
sets the variable, restarts, and nothing changes. The trail stays empty and
nothing says why. A recorder that is off while its operator believes it is on is
the failure the product exists to prevent.

How it was found

By contrast, not by testing. #139
added the same message to agentmetry detections and named the variable
correctly. Reviewing that commit is the only reason anyone read the older one.

The guard

A string comparison would pin this instance and miss the next. So
test_cli_env_var_names.py walks every string literal the CLI prints, extracts
anything shaped like an AGENTMETRY_ variable, and checks it against what the
package actually reads.

"Actually reads" needed both halves. The aliases declared on Settings are not
the whole set: AGENTMETRY_AGT_HMAC_KEY is read straight from os.environ at
the point of use and never passes through Settings. The first version of this
test flagged it as fake. Getting that right is what stops the guard from being
wrong in the opposite direction and training people to ignore it.

Verified both ways: the test passes on this branch, and reintroducing the old
name fails both assertions.

Checks

Nothing frozen is touched, fingerprint unchanged at 15846a0915769d4a. 1,192
tests pass, ruff clean, benchmark 0 missed and 0 false positives.