## Why

README.md currently mixes user-facing content (what kb does, how to install and use it) with developer-facing content (building from source, releasing, Docker image internals, architecture deep-dives). Users looking for quick-start instructions have to scroll past release scripts and build commands. Developers looking for contribution/build info have to hunt through user docs. Splitting these into README.md (users) and DEVELOPER.md (developers/contributors) follows standard open-source convention and makes both audiences' experience cleaner.

## What Changes

- **Trim README.md** to focus on user-facing content: what kb is, how to install (from pre-built images/releases), how to configure, how to use, engine configuration reference, data portability, and API reference.
- **Remove "from source" build instructions** from README.md (both engine and client sections).
- **Remove "Building and releasing" section** from README.md entirely.
- **Remove "Future: ROCm runtime migration"** developer note from README.md.
- **Create DEVELOPER.md** containing: building engine from source, building client from source, release process (client and engine), Docker image details, version checking, ROCm migration notes, and any other contributor-oriented content.
- **Add a link** from README.md to DEVELOPER.md for developers who want to build from source or contribute.

## Capabilities

### New Capabilities
- `developer-docs`: Developer-facing documentation covering building from source, releasing, and contributing.

### Modified Capabilities

(none - no spec-level behavior changes, this is a documentation restructuring)

## Impact

- **Files modified**: `README.md` (trimmed)
- **Files created**: `DEVELOPER.md` (new)
- **No code changes**: purely documentation restructuring
- **No API changes**: no functional impact