A schema's apply block is never validated: a typo silently disables the apply gate or blocks it forever #1867

Description

parseSchema checks every artifact's requires, but never the apply block.
openspec schema validate therefore reports a schema as valid when
apply.requires names an artifact id that does not exist, or when
apply.tracks names a file no artifact generates. Nothing reports the typo
later either:

1. Unknown apply.requires id: the apply gate turns off. The id is skipped,
   so apply reports ready and tells the agent "All required artifacts
   complete. Proceed with implementation." with only a proposal written.
2. Unknown apply.tracks path: apply blocks forever. Apply waits for a file
   nothing generates, and its remedy prints a literal <artifact> placeholder.
   Meanwhile openspec list counts a different file (tasks.md).

The code claims the opposite: src/commands/schema.ts:230-231 says "Dependency
graph validation is already done by parseSchema (it throws on cycles and
invalid references)".

Reproduction

A project schema copied from spec-driven (openspec/schemas/myflow), with
openspec/config.yaml set to schema: myflow.

A: typo in apply.requires. With apply: replaced by
requires: [desgin] and only proposal.md written:

openspec schema validate myflow
  ✓ Schema 'myflow' is valid
openspec instructions apply --change c --json
  state=ready missingArtifacts=None
  instruction=All required artifacts complete. Proceed with implementation.

The control, requires: [design], gives state=blocked missingArtifacts=['design'].

B: typo in apply.tracks. With requires: [tasks], tracks: task.md and
every artifact written:

openspec schema validate myflow
  ✓ Schema 'myflow' is valid
openspec status --change c --json        isPlanningComplete=True
openspec instructions apply --change c   state=blocked
  The task.md file is missing and must be created.
  Create it with `openspec instructions <artifact> --change c` ...
openspec list --json                     1/2 in-progress

Expected

schema validate rejects an apply.requires id or an apply.tracks path that
no artifact declares, the same way it rejects an invalid artifact requires.
docs/agent-contract.md says missingArtifacts "is what apply blocks on (the
schema's apply.requires)", so apply must never treat an unknown requirement
as satisfied.

Root cause

• src/core/artifact-graph/schema.ts: parseSchema runs
  validateRequiresReferences(schema.artifacts) and never looks at
  schema.apply.
• src/commands/workflow/instructions.ts: the missing-artifact check runs
  if (artifact && ...), so an unknown id counts as present, and
  collectMissingPrerequisites skips it with if (!artifact) continue.
• For tracks, findArtifactIdFor returns undefined, and the remedy falls
  back to the <artifact> placeholder.

Impact

Only projects with a custom schema are affected, but for them a one-character
typo either turns off the gate that stops an agent implementing before planning
is done, or leaves every change blocked behind a remedy that cannot be followed.
The command meant to catch it says the schema is valid.

Environment

• OpenSpec 1.13.0 (reproduced on the published npm package and on a build of
  main @ 9d4e597)
• Node v22, Linux