perf(rees): add cost-aware analyzer scheduling and profiles

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Context

REES currently treats requested analyzers too uniformly. Cheap local analyzers, package registry analyzers, and GitHub-heavy analyzers need different concurrency, timeout, and degradation behavior.

This issue adds a scheduler that plans analyzer execution around remaining budget, cost class, external fanout, and operator-selected profiles.

Parent: #1499

Deliverables

• Introduce analyzer cost classes such as local, registry, github-light, github-heavy, and tooling.
• Add per-class concurrency caps and per-host external call limits.
• Add request-level budget planning with a reserved response window so REES returns before the engine transport timeout.
• Add skip predicates so analyzers can be skipped before execution when relevant inputs are absent.
• Add profiles: fast, balanced, and deep.
• Preserve explicit REES_ANALYZERS behavior while allowing profile defaults.
• Emit safe per-analyzer status: ok, skipped, degraded, capped, timeout, elapsedMs, and safe reason codes.

Requirements

• Fast profile should favor local and low-cost checks.
• Balanced profile should remain the production default unless another issue explicitly changes that policy.
• Deep profile may run heavier checks but must still honor bounded budgets and degradation.
• No analyzer should consume the whole request budget without leaving time to serialize and return a partial response.
• Do not leak raw request payloads, diffs, prompts, comments, or tokens in diagnostics.

Acceptance criteria

• REES can return partial/degraded output before the caller timeout even when a heavy analyzer stalls.
• Expensive analyzers are capped by cost class and remaining budget.
• Skip predicates avoid calling registry/GitHub analyzers when no relevant files changed.
• Engine-side caller treats degraded/partial analyzer results as usable enrichment.

Validation

• Tests for scheduler ordering, skip predicates, profile selection, and reserved response budget.
• Regression test where a slow analyzer degrades without causing the whole request to abort.
• Focused engine enrichment-wire test for degraded REES responses.

Ownership

Maintainer-only. No gittensor:* labels.

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Closing as completed (2026-07-01 issue audit): Cost-aware analyzer scheduling shipped in #1833 (review-enrichment/src/scheduler.ts, REES_PROFILES).