test(cli): match the duplicate-name acceptance group to server auto-naming

`install-negatives › duplicate fleet name › second install … is rejected`
asserted behaviour the create path deliberately stopped having on Aug 18:
`create.zig` auto-suffixes a DEFAULTED name (`heroku_names.suffixed` → `-NNN`)
so a one-step install never dead-ends on a name nobody typed, and refuses only
an EXPLICIT `--name` with UZ-AGT-006. The test called the intended answer a
bug and had been red in every dev deploy since.

The group now proves both arms:

  - second defaulted install → exit 0, its own fleet id, stored name is the
    template name plus a three-digit tail;
  - explicit `--name` already in use → exit 3, UZ-AGT-006, and no id minted,
    which nothing covered before.

`parseInstallId` becomes `parseInstallEnvelope` + `installIdOf` so a test can
read the persisted `name` the 201 carries without parsing twice, and the
group's teardown reclaims the second copy as well as the first. Both
docstrings corrected to state the rule rather than the retired one.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>