feat: ship schemas for proposal, tasks, and design artifacts

[N4M3Z]

Every change artifact now validates against an embedded mdschema with the same repo-override path the spec schemas use. A proposal carries Why, What Changes, Capabilities, and Impact with status frontmatter. Tasks carry numbered sections of N.M checkboxes. A design carries its approach at warning severity, because existing designs predate the schema. Validation wires the schemas through validate_change, so tree and per-change runs check identically, and every repo with a docs spec tree inherits them. A wiring test pins the three diagnostic codes.

spec:none: the change adds validation schemas and no new lifecycle contract.

Note

Add Markdown schema validation for proposal, tasks, and design artifacts in changes

• Introduces three new schemas: proposal.mdschema, tasks.mdschema, and design.mdschema, each defining required headings, structure, and content rules for the corresponding artifact
• Embeds the schemas at compile time via new constants in templates.rs and bundles them into a ChangeSchemas struct in validate.rs\n- validate_change now reads proposal.md, tasks.md, and design.md from the change directory when present and emits diagnostics with codes proposal-schema-invalid, tasks-schema-invalid, and design-schema-invalid
• Adds a test in tests.rs stubbing the schema checker to verify the three diagnostic codes are produced for a sample change
• Behavioral Change: changes that include proposal.md, tasks.md, or design.md will now produce validation warnings or errors that did not exist before; existing canonical spec and delta validation behavior is unchanged

^{Macroscope summarized 87697fa.}

[macroscopeapp]

Approvability

Verdict: Not approved

Macroscope's review found this PR not approvable — This PR adds automatically active schema validation for proposal, task, and design artifacts, changing the diagnostics and possible outcomes of existing validation commands. The implementation is localized and fully author-owned, but the runtime behavior expansion merits human review.

Notes:

• Macroscope's correctness review did not run, so approvability was decided on eligibility alone.

^{You can add or adjust custom eligibility rules. Learn more.}

[runeseer]

🚫 [runeseer] _{reported by reviewdog 🐶}
High — proposal.mdschema rejects every proposal already in this repository's spec tree, so rune spec validate, archive preflight, and doctor now emit proposal-schema-invalid on artifacts this change does not touch.

• docs/openspec/changes/spec-compatibility/proposal.md:14 ### New Capabilities sits at depth 3, past max_depth: 2; decision-artifacts and development-lifecycle carry the same heading.
• docs/openspec/changes/decision-artifacts/proposal.md:1 starts at ## Why with no frontmatter, so the required status field is absent.

[runeseer]

Runeseer review — 1 open finding

Risk	Finding	Location
High	Proposal schema rejects all existing proposals in the spec tree	schemas/proposal.mdschema:15

Request changes. The new proposal schema errors on all three proposals already under docs/openspec/changes/, so validation fails on files this change never touches.

• The tasks and delta schemas match the existing artifacts; only the proposal schema conflicts.
• Fix by relaxing max_depth and the required status field, or by updating the three proposals in this change.
• Scaffolded proposals from templates/spec/proposal.md do satisfy the schema, so the break is limited to artifacts written before it.

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1 open · Reviewed 87697fa5 · review run · $1.21 · 2m40s · 34 turns

[runeseer]

@N4M3Z, the following findings block this head:

• schemas/proposal.mdschema:15: Proposal schema rejects all existing proposals in the spec tree

Choose one action:

• Fix all findings. Then apply review:runeseer to review the new head.
• Apply ignore:runeseer to accept the complete current verdict.