feat: migrate CLAUDE.md content to OpenSpec specs and initialize workflow

Move project description and tech stack research from CLAUDE.md into
openspec/specs/project/ and openspec/specs/stack/. Slim CLAUDE.md to a
pointer file. Populate config.yaml with project context. Add OpenSpec
CLI skills and commands.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

openspec/changes/archive/2026-04-03-migrate-claude-md-to-openspec/proposal.md

@@ -0,0 +1,27 @@
+## Why
+
+CLAUDE.md currently holds all project knowledge — description, constraints, and a large tech stack research block. With OpenSpec initialized, this content belongs in `openspec/specs/` where it can be managed as proper specs, referenced by changes, and won't conflict with CLAUDE.md's role as a slim workflow/instruction file.
+
+## What Changes
+
+- Extract project description and constraints into a `project` spec
+- Extract full technology stack research into a `stack` spec
+- Populate `openspec/config.yaml` with project context
+- Slim CLAUDE.md down to workflow instructions with pointers to OpenSpec
+- Remove stale GSD placeholder sections (conventions, architecture, profile)
+
+## Capabilities
+
+### New Capabilities
+- `project`: Project identity, core value statement, and non-negotiable constraints
+- `stack`: Technology stack decisions — packages, versions, alternatives, patterns, compatibility, and sources
+
+### Modified Capabilities
+<!-- None — no existing specs yet -->
+
+## Impact
+
+- `CLAUDE.md`: Reduced from ~147 lines to a slim pointer file
+- `openspec/specs/project/spec.md`: New file with project identity
+- `openspec/specs/stack/spec.md`: New file with stack research
+- `openspec/config.yaml`: Updated with project context