docs(01): capture phase context

Phase 01: Foundation
- Container isolation decisions documented
- Build strategy established
- Dev workflow defined

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.planning/phases/01-foundation/01-CONTEXT.md

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+# Phase 1: Foundation - Context
+
+**Gathered:** 2026-02-03
+**Status:** Ready for planning
+
+<domain>
+## Phase Boundary
+
+Go backend running in Docker with isolated storage. Container serves HTTP and has access only to the mounted data volume. This phase establishes the infrastructure foundation — no application logic yet.
+
+</domain>
+
+<decisions>
+## Implementation Decisions
+
+### Container Isolation
+- Standard isolation — volume mount restriction only, no hardened filesystem
+- Port 32768 inside container — high port to avoid collisions with other Pi services
+- Data volume mounted at `/data` inside container
+- HTTP only — no outbound network access needed
+
+### Build Strategy
+- Multi-arch image using docker buildx (x86_64 + ARM64)
+- Single Dockerfile — no separate dev/prod files
+- Base image: Debian slim (compatibility and debugging over minimal size)
+- Push to local Gitea registry (once set up)
+
+### Dev Workflow
+- Docker Compose for local development
+- Deploy to Pi: git pull from Gitea + build image directly on Pi
+- Go version: Latest stable (1.22+)
+
+### Claude's Discretion
+- Go project structure and module naming
+- Hot reload / watch mode setup for local dev (or manual rebuild)
+- Multi-stage Dockerfile optimization
+- Health check endpoint design
+
+</decisions>
+
+<specifics>
+## Specific Ideas
+
+- User plans to move primary dev environment to Pi after setting up local Gitea, so cross-platform dev experience is transitional
+- Pi may run other services, hence the high port (32768) to avoid conflicts
+
+</specifics>
+
+<deferred>
+## Deferred Ideas
+
+None — discussion stayed within phase scope
+
+</deferred>
+
+---
+
+*Phase: 01-foundation*
+*Context gathered: 2026-02-03*