docs: scope and trim agent guidance

[BeatrixKat]

Summary

• reduce the always-loaded root AGENTS.md from about 75 KB to about 11 KB
• keep cross-cutting invariants and contribution policy in the root guide
• move tests, plugins, skills, optional skills, TUI, and CLI guidance into scoped AGENTS.md files
• add a compact component-routing guide under docs/development

Why

The gateway currently truncates AGENTS.md at 65,280 characters. Besides losing instructions unpredictably, the monolithic guide loads detailed subsystem tutorials into every agent context. Scoped project files keep the relevant guidance close to the code while restoring context headroom.

Validation

• rebased onto current upstream main
• git diff --check passes
• root AGENTS.md is 11,057 characters
• local gateway restart loaded the trimmed guide without a new truncation warning
• documentation-only change; no runtime code modified

[alt-glitch]

This was generated by AI during triage.

Related: #61460, #53996, and #64206 are open alternatives for reducing always-loaded AGENTS.md context. This PR uses scoped subtree guides plus a component-routing guide; the overlapping documentation policy needs a maintainer choice rather than a duplicate classification.

[BeatrixKat]

Closing because this change was intended only for the owner's local Mac mini installation, not as an upstream Hermes contribution.