feat(failure): distinguish quota-exceeded from the rest of FailureTerminal

[scttfrdmn]

Summary

• New pkg/failure.IsQuotaExceeded(err) bool — a finer-grained question than ClassifyFailure's FailureTerminal, which also covers bad AMI/IAM/malformed-request errors this correctly reports false for.
• quotaErrorCodes (InstanceLimitExceeded, VcpuLimitExceeded, MaxSpotInstanceCountExceeded) split out of terminalErrorCodes into its own map, checked first in ClassifyFailure — so each code is listed exactly once, not duplicated across two maps.
• pkg/watcher.IsQuotaExceeded aliases it, matching the existing alias pattern for every other pkg/failure export.
• pkg/snipe.Status gains QuotaExceeded bool, set on every Options.Progress report that carries a LastErr — so a caller doesn't need to separately import pkg/failure and re-classify the error itself.

No default behavior changes. A quota error still classifies as FailureTerminal and Snipe still returns immediately on it, exactly as before — TestClassifyFailure, TestSnipe_TerminalStopsImmediately, etc. all pass unmodified, confirming the classification itself didn't move. This is purely an additive signal for a caller building its own backoff/reduce-concurrency logic on top.

Real-world motivation: several concurrent pkg/snipe.Snipe calls against the same account (one per fleet shard) each get MaxSpotInstanceCountExceeded and give up immediately when the account's own quota is momentarily saturated by the caller's OTHER in-flight requests — indistinguishable, before this change, from a hard account wall needing a support ticket.

Fixes #116.

Test plan

• TestIsQuotaExceeded (11 cases) — all 3 quota codes report true; every other terminal code, every capacity code, an unknown code, and a plain non-AWS error all report false
• TestIsQuotaExceeded_StillClassifiesTerminal — explicit regression guard that ClassifyFailure is unchanged for the 3 quota codes
• TestSnipe_ProgressReportsQuotaExceeded / TestSnipe_ProgressDoesNotReportQuotaForOrdinaryTerminal — Status.QuotaExceeded correctly true/false via the real retry loop
• watcher.TestIsQuotaExceeded — alias reaches the real classifier
• Existing TestClassifyFailure (both pkg/failure and pkg/watcher copies) pass unmodified — proves quota codes still classify as FailureTerminal
• go build ./..., go vet ./..., gofmt -l clean (root module + lambda/capacity-poller); full go test ./... green
• golangci-lint run — 2 pre-existing issues in untouched files (pkg/watcher/notifier.go, pkg/watcher/store.go), zero in touched files
• make check-docs — no drift

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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