fix: merge instructions arrays across config files

[elithrar]

The instructions config array was being replaced instead of merged when combining global and project configs. This meant global instructions (e.g. from ~/.config/opencode/opencode.json) were lost when a project had its own instructions array.

this PR:

• renames mergeConfigWithPlugins to mergeConfigConcatArrays to reflect its broader purpose
• adds instructions array merging alongside the existing plugin array handling
• adds tests for instructions merging and deduplication

The plugin array already had this fix (PR #4724), but instructions was missed.

Paths are deduplicated by their raw string value, not their resolved location. Resolving paths at merge time isn't feasible since globs need filesystem access and project context isn't fully established yet.

Config location	instructions value	Resolves to
~/.config/opencode/opencode.json	["INSTRUCTIONS.md"]	Looks in project dir (not global config dir)
~/.config/opencode/opencode.json	["~/.config/opencode/INSTRUCTIONS.md"]	Correctly resolves to global file
./opencode.json	["INSTRUCTIONS.md"]	Looks in project dir

Users should use absolute or ~/ paths in global config to ensure they resolve correctly.

[elithrar]

before fix:

$ opencode debug config | jq '.instructions'
[
  "STYLE_GUIDE.md"
]

after:

$ cd packages/opencode && bun run build

$ ./packages/opencode/dist/opencode-darwin-arm64/bin/opencode debug config | jq '.instructions'
[
  "/Users/matt/.config/opencode/INSTRUCTIONS.md",
  "STYLE_GUIDE.md"
]