feat(scheduler): add bounded ownership-aware task planning

[dporkka]

Summary

Add a pure, reusable scheduler primitive for multi-task development-agent execution.

Semantics

• dependency DAG validation
• conservative repository path-prefix ownership conflicts
• host CPU and memory capacity
• maximum parallel task count
• existing running-task resource/ownership claims
• failed dependencies skip dependents while unrelated work remains runnable
• deterministic dry-run wave planning

This intentionally does not modify the existing worker or budget engine yet. It establishes a low-blast-radius scheduling primitive that can be composed with current Engine.CheckRun budget enforcement and workspace provisioning in a follow-up.

Exact-head verification

• cd packages/scheduler && go test ./... — pass
• cd packages/scheduler && go test -race ./... — pass
• cd packages/scheduler && go vet ./... — pass
• GitNexus impact review: existing GetConcurrentRuns has one direct caller (CheckRun); neither symbol was modified
• GitNexus detect-changes: new package/workspace registration overlaps no existing indexed symbols
• branch flattened to one commit directly on current main

CI infrastructure note

GitHub Actions run 36456250850 failed before runner allocation. Lint/Test/Build each reported zero steps and no log blob. A manual retry created new job IDs and failed identically with zero steps/logs, so this is not executable code-validation evidence.

Broader local gate

make test-packages passes through agents, crypto, db, events, gateway, models, policies, prfactory, repo-intel, and reviewer, then hits the existing packages/runtimes tmpfs cleanup failure (device or resource busy). The scheduler package itself is green and does not modify runtimes.

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