fix(scheduler): terminalize permanently invalid targets

[mnkj0021]

Fixes #1395

Summary

• terminalize pending requests pinned to deleted agents or missing/retired exact agent versions
• preserve pending recovery for missing agents, temporary agent unavailability, queue-less targets, and versionless requests
• keep queue-movement recovery intact and never substitute a different active agent version
• persist a clear failure message plus the existing machine-readable agent_deleted / agent_version_unavailable error codes
• add scheduler regression coverage for permanent invalidity, temporary unavailability, queue movement, and exact-version affinity

Validation

• corepack pnpm --filter scheduler build — passed on a fresh recheck
• corepack pnpm exec vitest run apps/scheduler/src/request-scheduler.test.ts — 18/18 passed
• git diff --check — passed
• upstream Scope CI, lint, build, unit tests, gitleaks, notice and CLA checks are green
• branch is current with upstream main (0 commits behind)

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