feat: establish canonical task dependency reads

[andy-esch]

Summary

• Add canonical task depends_on persistence and schema/JSON projection for repository-global prerequisites.
• Build an immutable strict task-graph snapshot with explicit healthy, degraded, and broken health; SCC-based cycle attribution; derived lifecycle/gate state; sound completion; topology; downstream impact; and deterministic blocker paths.
• Separate full causal blockers from the action-oriented blocking frontier, and keep authorization tied to derived Eligible state rather than blocker-list emptiness.
• Diagnose legacy blocked_by, dependencies, and blocks references without migrating them. Exactly resolved, structurally safe references are lint advisories with exit zero; missing, ambiguous, self-referential, or cyclic projections remain errors.
• Prevent first-party bypasses: generic create/set/edit and lint --fix cannot add, remove, or reinterpret graph-owned dependency fields.
• Preserve source identity for unreadable and duplicate-ID records, including path-faithful lint attribution and distinct missing/unreadable/invalid-reference blocker reasons.

Rollout boundary

This is the dependency read and validation foundation. It intentionally does not add public dependency or Thread mutation commands yet.

depends_on is preservation-only until the guarded task depend add/remove slice lands. task set --force does not bypass graph ownership, malformed dependency frontmatter fails closed, and lint --fix reports rather than normalizes a would-be graph repair.

The next implementation slice is the portable store-owned repository mutation boundary, followed by guarded dependency commands and queries.

Design and adversarial review

• Amend ADR-0006 with the implementation-readiness and dependency-foundation contracts.
• Record and close the independent Gemini audit and Claude audit.
• Replace the original back-edge cycle discovery with SCC membership after both audits produced intersecting-cycle counterexamples.
• Remove the speculative one-implementation graph adapter seam. The bounded library bake-off did not justify a project dependency; direct taskflow-owned adversarial tests remain in-repository.
• Track the remaining non-blocking follow-ups on the concrete downstream tasks: machine-readable field ownership and graph-depth thresholds on 6g3q4rt7mgjn, and canonical snapshot-loader consolidation on 6g3q4rt0wzkq.

Documentation

• Explain the preservation-only dependency rollout in the README and task authoring schema.
• Document TaskGraph and graph/store ownership in the architecture guide.
• Expand generated lint, task set, and task edit references with dependency guardrails and advisory/error behavior.

Validation

• go test ./...
• go test -race ./...
• golangci-lint run ./... — zero issues
• go vet ./...
• go mod tidy -diff
• Generated CLI docs, schema comments, and golden machine contracts are current
• git diff --check
• Both audit lints pass with no open findings
• Live planning lint emits exactly six safe legacy advisories, zero unreadable files, and exits zero