feat: add structured spec discovery step to proposal instruction

Replace vague "Check openspec/specs/ for existing spec names" with a
concrete CLI-based discovery step using `openspec list --specs --json --detail`.

This gives the AI a repeatable workflow for identifying New vs Modified
capabilities before writing the Capabilities section, with a graceful
fallback for environments where --detail is not yet available.

Closes #901

Author: sh940701

schemas/spec-driven/schema.yaml

@@ -13,12 +13,13 @@ artifacts:
       - **Why**: 1-2 sentences on the problem or opportunity. What problem does this solve? Why now?
       - **What Changes**: Bullet list of changes. Be specific about new capabilities, modifications, or removals. Mark breaking changes with **BREAKING**.
       - **Capabilities**: Identify which specs will be created or modified:
-        - **New Capabilities**: List capabilities being introduced. Each becomes a new `specs/<name>/spec.md`. Use kebab-case names (e.g., `user-auth`, `data-export`).
-        - **Modified Capabilities**: List existing capabilities whose REQUIREMENTS are changing. Only include if spec-level behavior changes (not just implementation details). Each needs a delta spec file. Check `openspec/specs/` for existing spec names. Leave empty if no requirement changes.
+        - Before filling this in, discover existing specs by running `openspec list --specs --json --detail`. Review each spec's `overview` to understand what it covers. If `--detail` is unavailable, browse `openspec/specs/` and read each spec's Purpose section instead.
+        - **New Capabilities**: List capabilities being introduced that don't overlap with any existing spec. Each becomes a new `specs/<name>/spec.md`. Use kebab-case names (e.g., `user-auth`, `data-export`).
+        - **Modified Capabilities**: List existing capabilities whose REQUIREMENTS are changing. Only include if spec-level behavior changes (not just implementation details). Each needs a delta spec file. Use the `id` from the catalog output above. Leave empty if no requirement changes.
       - **Impact**: Affected code, APIs, dependencies, or systems.
 
       IMPORTANT: The Capabilities section is critical. It creates the contract between
-      proposal and specs phases. Research existing specs before filling this in.
+      proposal and specs phases. Always run spec discovery before filling this in.
       Each capability listed here will need a corresponding spec file.
 
       Keep it concise (1-2 pages). Focus on the "why" not the "how" -

src/core/templates/workflows/continue-change.ts

@@ -100,6 +100,7 @@ Common artifact patterns:
 
 **spec-driven schema** (proposal → specs → design → tasks):
 - **proposal.md**: Ask user about the change if not clear. Fill in Why, What Changes, Capabilities, Impact.
+  - Before writing Capabilities, run \`openspec list --specs --json --detail\` to discover existing specs. Compare each spec's overview against the proposed change to determine New vs Modified capabilities.
   - The Capabilities section is critical - each capability listed will need a spec file.
 - **specs/<capability>/spec.md**: Create one spec per capability listed in the proposal's Capabilities section (use the capability name, not the change name).
 - **design.md**: Document technical decisions, architecture, and implementation approach.
@@ -219,6 +220,7 @@ Common artifact patterns:
 
 **spec-driven schema** (proposal → specs → design → tasks):
 - **proposal.md**: Ask user about the change if not clear. Fill in Why, What Changes, Capabilities, Impact.
+  - Before writing Capabilities, run \`openspec list --specs --json --detail\` to discover existing specs. Compare each spec's overview against the proposed change to determine New vs Modified capabilities.
   - The Capabilities section is critical - each capability listed will need a spec file.
 - **specs/<capability>/spec.md**: Create one spec per capability listed in the proposal's Capabilities section (use the capability name, not the change name).
 - **design.md**: Document technical decisions, architecture, and implementation approach.