fix(config): handle null rules field in project config (Fission-AI#529)

* feat(config): add project-level configuration via openspec/config.yaml

Adds openspec/config.yaml support for project-level customization without
forking schemas. Teams can now:

- Set a default schema (used when --schema flag not provided)
- Inject project context into all artifact instructions
- Add per-artifact rules (e.g., proposal rules, specs rules)

Key changes:
- New `src/core/project-config.ts` with Zod schema and resilient parsing
- New `src/core/config-prompts.ts` for interactive config creation
- Updated schema resolution order: CLI → change metadata → config → default
- Updated instruction generation to inject <context> and <rules> XML sections
- Integrated config creation prompts into `artifact-experimental-setup`

Schema resolution precedence:
1. --schema CLI flag (explicit override)
2. .openspec.yaml in change directory (change-specific)
3. openspec/config.yaml schema field (project default)
4. "spec-driven" (hardcoded fallback)

* test(config): add e2e tests and performance benchmarks for project config

- Add project config integration tests in artifact-workflow.test.ts:
  - Test new change uses schema from config
  - Test CLI schema overrides config schema
  - Test context and rules injection in instructions
  - Test backwards compatibility without config file
  - Test immediate reflection of config changes

- Add performance benchmark tests in project-config.test.ts:
  - Typical config (1KB): <20ms target
  - Large config (50KB): <50ms target
  - Repeated reads consistency check
  - Missing config fast-path test

- Add project configuration documentation in experimental-workflow.md:
  - Config fields reference (schema, context, rules)
  - Schema precedence explanation
  - Artifact IDs by schema
  - Troubleshooting guide

- Mark all tasks complete in tasks.md

* refactor(config): remove benchmark tests, document decision in source

Remove performance benchmark tests from project-config.test.ts and
document the results directly in the source code instead. Benchmarks
showed config reads are fast enough (~0.5ms typical) that caching
is unnecessary.

* fix(config): resolve three issues in project config feature

- Remove nested <template> tags: generateInstructions() no longer wraps
  template content since printInstructionsText() already handles XML
  structure
- Fix schema message accuracy: newChangeCommand() now uses the resolved
  schema returned from createChange() instead of hardcoded fallback
- Add TTY check: artifact-experimental-setup skips interactive prompts
  in non-TTY environments (CI, automation) to prevent hangs

* fix(config): handle null rules field in project config

Add null check when parsing rules field since YAML `rules:` with no
value parses to null, and `typeof null === 'object'` in JavaScript.
Without this check, Object.entries(null) would throw an error.

Author: TabishB

src/core/project-config.ts

@@ -117,8 +117,8 @@ export function readProjectConfig(projectRoot: string): ProjectConfig | null {
     if (raw.rules !== undefined) {
       const rulesField = z.record(z.string(), z.array(z.string()));
 
-      // First check if it's an object structure
-      if (typeof raw.rules === 'object' && !Array.isArray(raw.rules)) {
+      // First check if it's an object structure (guard against null since typeof null === 'object')
+      if (typeof raw.rules === 'object' && raw.rules !== null && !Array.isArray(raw.rules)) {
         const parsedRules: Record<string, string[]> = {};
         let hasValidRules = false;
 

test/core/project-config.test.ts

@@ -142,6 +142,30 @@ rules: ["not", "an", "object"]
         );
       });
 
+      it('should handle rules: null without aborting config parsing', () => {
+        // YAML `rules:` with no value parses to null
+        const configDir = path.join(tempDir, 'openspec');
+        fs.mkdirSync(configDir, { recursive: true });
+        fs.writeFileSync(
+          path.join(configDir, 'config.yaml'),
+          `schema: spec-driven
+context: Valid context
+rules:
+`
+        );
+
+        const config = readProjectConfig(tempDir);
+
+        // Should still parse schema and context despite null rules
+        expect(config).toEqual({
+          schema: 'spec-driven',
+          context: 'Valid context',
+        });
+        expect(consoleWarnSpy).toHaveBeenCalledWith(
+          expect.stringContaining("Invalid 'rules' field")
+        );
+      });
+
       it('should filter out invalid rules for specific artifact', () => {
         const configDir = path.join(tempDir, 'openspec');
         fs.mkdirSync(configDir, { recursive: true });