auth status: explain why authentication failed instead of a bare "Unauthorized"

[jordanenglish]

What

tfctl auth status currently prints a single opaque line — X Unauthorized for <host> — for three genuinely different failures:

1. No token configured at all
2. A token the server rejected (HTTP 401)
3. The request never reached the server (network/DNS, or some other non-2xx HTTP status)

You can't tell an expired token from a network blip, and the message points at no remedy.

Change

Classify the /account/details error and print a cause-specific, actionable message. The go-tfe *APIError arrives wrapped in a *url.Error, so classification uses errors.As to walk the chain rather than matching the top-level type.

Cause	Message
no token	No token configured for <host>. Run 'tfctl auth login' to authenticate.
rejected (401)	token may be expired/revoked (tfctl auth login), or an SSO session that has lapsed on SSO-protected Terraform Enterprise (re-authenticate in the browser, then retry)
other HTTP status	<host> returned HTTP N (not an authentication problem).
unreachable	Could not reach <host>: <err>

JSON/agent output gains a machine-readable reason field (no_token / rejected / server_error / unreachable) so scripts can branch on the cause.

The 401 message is the motivating case: on SAML-SSO Terraform Enterprise a user token is gated by the SSO session, so it 401s well before its own expiry — indistinguishable from a truly expired token under the old wording.

Tests

• Updated TestStatus_Unauthorized / TestStatus_NoToken to assert the new cause-specific messages (drives a real 401 through the go-tfe client).
• Added TestStatus_Unauthorized_JSON (asserts reason in JSON output) and TestClassifyAuthError (covers the 401 / wrapped-*url.Error / other-status / unreachable branches).
• gofmt, go vet, and go test ./... clean.

PR Checklist

• Run npx changie new or install changie to prepare a new changelog entry for the next set of release notes.
  • Added .changes/unreleased/ENHANCEMENTS-*.yaml (kind: ENHANCEMENTS).
• Ensure any command changes are sensitive to these global flags:
  • --json — Force machine readable output to stdout. Does not apply to stderr.
  • --markdown — Force markdown output to stdout. Does not apply to stderr.
  • --dry-run — Don't make any actual writes or other mutations. Describe what would have changed to stderr.
  • --quiet — Don't render output to stdout.
  • auth status is read-only, so --dry-run is not applicable. The failure messages go to stderr (as before); JSON/agent output gains a machine-readable reason field on the existing inactive result. --quiet behavior is unchanged.
• Get the logging interface from the context and add debug logging for interesting conditions and nonfatal situations.
  • No new silent nonfatal paths: the previously opaque failure causes are now surfaced to the user on stderr.
• Run make gen/screenshot if the root command output changes.
  • Not required: only auth status output changes; the root command output is unchanged.
• Add the Autocomplete field to positional arguments and flags to assist shell autocomplete.
  • N/A: no arguments or flags are added.

PCI review checklist

• I have documented a clear reason for, and description of, the change I am making.

• If applicable, I've documented a plan to revert these changes if they require more than reverting the pull request.

  • Reverting the PR fully removes the change; no additional revert steps or data migration.

• If applicable, I've documented the impact of any changes to security controls.

  • N/A: this only improves diagnostic messaging for an existing auth failure. No auth, token handling, or permission behavior changes, and no secret material (e.g. token value or fingerprint) is printed.

[brandonc]

Once again, a very useful and uncontroversial change. Thanks!

[brandonc]

Re-opened as #94 because I had to fix a test and a suggestion formatting issue