CLAUDE.md/AGENTS.md instruction-file loading reworked: caching, nested discovery, and 'none' mode

What

The pipeline that loads CLAUDE.md and related project-instruction files was reworked in several ways:

• Loading now computes the set of instruction files first and compares it to the previous version, reusing cached CLAUDE.md text when the instruction files haven't changed, instead of always recomputing.
• Instruction-file discovery was rewritten to resolve a session root and walk parent directories for nested CLAUDE.md/AGENTS.md files, deduping files by both path and trimmed content across project, local, user, and memory sources.
• The projectInstructions setting's none mode now behaves differently: it drops only the project's own and your personal instruction files, while your organization's managed CLAUDE.md and memory still load. Previously, none dropped the managed CLAUDE.md too.

Why

These changes make instruction-file loading faster (via caching), more thorough (finding nested files in parent directories), and change what none actually excludes — organizations relying on managed CLAUDE.md should note it now stays loaded even when a user sets projectInstructions to none.