docs(adr): add Percy automation ADR

docs/adr/0002-percy-automation.md

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+# 2. Percy Automation
+
+Date: 2026-02-12
+
+## Status
+
+Accepted
+
+## Context
+
+We need automated visual regression testing to catch unintended UI changes. Previously, Percy snapshots were executed manually with ~990 snapshots total, which was paused due to high costs and slow execution times. Running the full snapshot suite for every PR was not sustainable.
+
+## Decision
+
+We will automate Percy visual regression testing through GitHub Actions with intelligent partial builds:
+
+1. **Partial builds for PRs**: Execute only snapshots related to changed files by:
+   - Detecting changed components from git diff against main branch
+   - Using the existing `component-metadata.json` mappings (component and submodules fields) to determine snapshot dependencies
+   - Triggering full builds only when global files change (tokens, variables, mixins)
+
+2. **Full builds for main commits**: Execute and auto-approve the complete snapshot suite on the main branch
+
+3. **Simplified dependency tracking**: Re-use existing component metadata instead of automated code analysis (e.g., mixin dependency detection), as the additional complexity provides minimal benefit
+
+## Consequences
+
+**Positive:**
+- Significantly reduced Percy build times and costs for PRs by running only relevant snapshots
+- Automated workflow reduces manual effort and human error
+- Catches visual regressions before merge
+- Full coverage maintained on main branch
+
+**Negative:**
+- Dependency mapping must be manually maintained in `component-metadata.json`
+- Risk of missing visual regressions if dependencies are not accurately mapped
+- Global changes (tokens, variables, mixins) still trigger full builds