Handle transient verify and retry states cleanly

[jclusso]

• treat HTTP 249 as a retryable pending state with try_again code
• map try_again to the retry-later exit bucket
• align rate-limit backoff with documented RateLimit-Reset timestamp
• route auth refresh/progress through command context and stderr
• fix quiet API-key login output and reset root flag state between runs
• add regression coverage for retry, pending, quiet, and command-state behavior

[Copilot]

Pull request overview

Cleans up handling of transient API responses (HTTP 249, 429) and a few command-context plumbing issues. Adds try_again as a new error code mapped to exit code 3, switches the rate-limit backoff to use the RateLimit-Reset Unix timestamp, threads cmd.Context() through requireAuth, routes spinner/progress output to stderr, fixes quiet-mode for API-key login, and resets root flag globals between runs. Documentation and tests are updated to match.

Changes:

• Treat HTTP 249 as a retryable pending state with new try_again code, mapped to the retry-later exit bucket.
• Reinterpret RateLimit-Reset as a Unix timestamp (drop Retry-After fallback) and align human/JSON renderers and backoff accordingly.
• Plumb cmd.Context() through requireAuth, send spinner/progress to stderr, honor Quiet on API-key login, and reset root flag globals on each newRootCmd.

Reviewed changes

Copilot reviewed 16 out of 16 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
README.md	Documents 249/try_again, transient retry, and Reset-as-timestamp semantics.
internal/api/errors.go	Updates RateLimit.Reset doc to Unix timestamp.
internal/api/client.go	Adds isRetryableStatus (249/429), reworks backoffFor to use Reset timestamp + exponential fallback, drops Retry-After.
internal/api/client_test.go	Adds tests for 249 error, backoff timestamp behavior, and option forwarding.
cmd/errors.go	Adds try_again code, 249 "Pending:" rendering, and timestamp-based 429 retry hint.
cmd/errors_test.go	Tests new 249 rendering, JSON code, and exit-code mapping.
cmd/context.go	requireAuth now takes a context.Context for refresh.
cmd/context_test.go	Updates callers to pass context.
cmd/account.go, cmd/batch.go, cmd/verify.go	Pass cmd.Context() to requireAuth; route progress/spinner to stderr.
cmd/batch_e2e_test.go	Comment touch-up.
cmd/login.go	Honors ctx.Quiet for API-key login output.
cmd/login_e2e_test.go	Adds quiet-mode regression test.
cmd/root.go	Adds resetRootFlagState() invoked from newRootCmd.
cmd/root_test.go	Verifies flag globals are reset.

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