docs: rewrite stale CLAUDE.md sections to match current code (audit #9)

[sbalabanov]

Summary

CLAUDE.md described a deleted generation of the error, metrics, caching, and cancellation systems. This PR rewrites every stale section to match the code on main today; every symbol and file path named in the doc has been verified against the source.

• Error Classification: replaced core/common, ClassifiedError, common.WithReason, ErrorTypeUser/ErrorTypeInfra, failureReason* constants with the real core/errors system (TangoError, NewUser/NewInfra/NewInfraRetryable, GetErrorCode, errors.Fields); fixed storage.NotFoundError to storage.ErrNotFound + IsNotFound()
• Controllers: replaced stale calls/success/failure counter pattern with metrics.Begin start counter + op.Complete finish histogram tagged result, plus failures{error_code} counter
• Orchestrator: replaced common.WithReason(reason, errorType, err) with the real per-cause classifiers (classifyLeaseError, classifyGitError, classifyBazelClientError)
• Caching: moved helpers from core/common to core/cachekey; replaced blanket best-effort claim with the two-tier policy (identity-bearing reads fail-fast, compared-targets best-effort); documented controller's read-through caching role
• Cancellation: replaced context.WithoutCancel with the real appCtx pattern
• Project Layout: added entity/, mapper/, internal/, observability/metrics/, integration/, docs/, core/cachekey/, core/errors/; removed nonexistent common
• Makefile: relaxed alphabetical-sort + auto-help claim to match reality (hand-written help)
• Make Targets: added missing bench, test-integration, cover, lint, version

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