fix(install): resolve non-.md context paths like paths.json

[darrenhinde]

Summary

• resolve_component_path() in install.sh was unconditionally appending .md to any context ID containing a /, so context:core/config/paths.json was looked up as .opencode/context/core/config/paths.json.md — a path that doesn't exist.
• This caused the installer to emit ⚠ Could not find path for context:core/config/paths.json and count the component as failed, even though the file is present in both the repo and the registry.
• Fix: try the .md path first (preserving existing behaviour for all markdown context files), then fall back to the bare path for non-markdown files (e.g. .json).

Root Cause

# Before — always appended .md, broke .json context files
jq_exec "first(.components.contexts[]? | select(.path == \".opencode/context/${component_id}.md\") | .path)" ...

Fix

# After — .md first, then bare path fallback
result=$(jq_exec "first(.components.contexts[]? | select(.path == \".opencode/context/${component_id}.md\") | .path)" ...)
if [ -z "$result" ] || [ "$result" = "null" ]; then
    result=$(jq_exec "first(.components.contexts[]? | select(.path == \".opencode/context/${component_id}\") | .path)" ...)
fi

Testing

Verified with jq directly against registry.json:

• Old lookup (.md appended) → empty (broken)
• New fallback (bare path) → ".opencode/context/core/config/paths.json" ✅
• Normal .md context (core/config/navigation) → still resolves correctly via first branch ✅

Closes #251