AGENTS.md/CLAUDE.md instruction loading revamped: instructionFiles setting, new modes, plugin file stats

What

• The setting is renamed from projectInstructions to instructionFiles; the old name still works as a deprecated alias but logs a one-time warning telling you to migrate.
• The recognized modes changed from claude, agents-fallback, both, none to claude-md, claude-md-or-agents-md, claude-md-and-agents-md, managed-only.
• The default mode is now claude-md-or-agents-md, so a project with no CLAUDE.md of its own now automatically gets its AGENTS.md files loaded instead, without needing to turn anything on.
• New telemetry events (agents_md_load, agents_md_nested) record file count, import count, total content length, and whether the file walk succeeded or failed, tagged by the resolved mode.
• Project context-loading results now always include a pluginInstructionFiles field (with fileCount and totalContentLength) alongside the existing claudeMd, instructionFiles, and memoryFiles fields.

Why

This clarifies and simplifies how Claude Code decides which project instruction files (CLAUDE.md, AGENTS.md) to load, makes AGENTS.md usable out of the box for projects that don't have their own CLAUDE.md, and gives better visibility into which instruction files, including ones contributed by plugins, actually got loaded.