feat(config): add project-level configuration via openspec/config.yaml (Fission-AI#499)

* 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

Author: TabishB

docs/experimental-workflow.md

@@ -78,6 +78,98 @@ openspec artifact-experimental-setup
 
 This creates skills in `.claude/skills/` that Claude Code auto-detects.
 
+During setup, you'll be prompted to create a **project config** (`openspec/config.yaml`). This is optional but recommended.
+
+## Project Configuration
+
+Project config lets you set defaults and inject project-specific context into all artifacts.
+
+### Creating Config
+
+Config is created during `artifact-experimental-setup`, or manually:
+
+```yaml
+# openspec/config.yaml
+schema: spec-driven
+
+context: |
+  Tech stack: TypeScript, React, Node.js
+  API conventions: RESTful, JSON responses
+  Testing: Vitest for unit tests, Playwright for e2e
+  Style: ESLint with Prettier, strict TypeScript
+
+rules:
+  proposal:
+    - Include rollback plan
+    - Identify affected teams
+  specs:
+    - Use Given/When/Then format for scenarios
+  design:
+    - Include sequence diagrams for complex flows
+```
+
+### Config Fields
+
+| Field | Type | Description |
+|-------|------|-------------|
+| `schema` | string | Default schema for new changes (e.g., `spec-driven`, `tdd`) |
+| `context` | string | Project context injected into all artifact instructions |
+| `rules` | object | Per-artifact rules, keyed by artifact ID |
+
+### How It Works
+
+**Schema precedence** (highest to lowest):
+1. CLI flag (`--schema tdd`)
+2. Change metadata (`.openspec.yaml` in change directory)
+3. Project config (`openspec/config.yaml`)
+4. Default (`spec-driven`)
+
+**Context injection:**
+- Context is prepended to every artifact's instructions
+- Wrapped in `<context>...</context>` tags
+- Helps AI understand your project's conventions
+
+**Rules injection:**
+- Rules are only injected for matching artifacts
+- Wrapped in `<rules>...</rules>` tags
+- Appear after context, before the template
+
+### Artifact IDs by Schema
+
+**spec-driven** (default):
+- `proposal` — Change proposal
+- `specs` — Specifications
+- `design` — Technical design
+- `tasks` — Implementation tasks
+
+**tdd**:
+- `spec` — Feature specification
+- `tests` — Test file
+- `implementation` — Implementation code
+- `docs` — Documentation
+
+### Config Validation
+
+- Unknown artifact IDs in `rules` generate warnings
+- Schema names are validated against available schemas
+- Context has a 50KB size limit
+- Invalid YAML is reported with line numbers
+
+### Troubleshooting
+
+**"Unknown artifact ID in rules: X"**
+- Check artifact IDs match your schema (see list above)
+- Run `openspec schemas --json` to see artifact IDs for each schema
+
+**Config not being applied:**
+- Ensure file is at `openspec/config.yaml` (not `.yml`)
+- Check YAML syntax with a validator
+- Config changes take effect immediately (no restart needed)
+
+**Context too large:**
+- Context is limited to 50KB
+- Summarize or link to external docs instead
+
 ## Commands
 
 | Command | What it does |

openspec/changes/project-config/.openspec.yaml

@@ -0,0 +1,2 @@
+schema: spec-driven
+created: "2025-01-13"