docs: split AGENTS.md's least-frequently-needed sections into docs/ references

[lmktechnology]

Summary

• AGENTS.md was carrying its full state/ directory inventory, agent-only reference-skill trigger table, and Relay section inline, pushing the file to 576 lines / ~23k tokens of always-loaded context.
• Moved that detail into three new reference docs, replacing the inline content in AGENTS.md sections 2, 13, and 14 with one-line pointers, per the file's own progressive-disclosure convention (it already does this for docs/configuration.md, docs/herdr-backend.md, etc.).
• Added the new docs to README's documentation index.

Changes

• docs/state-directory-layout.md (new) - full state/ field-by-field inventory
• docs/agent-only-reference-skills.md (new) - full agent-only reference-skill trigger list
• docs/relay.md (new) - full Relay activation/wake-handling/public-reply contract
• AGENTS.md - sections 2, 13, 14 now point to the above instead of inlining them (576 -> 508 lines)
• README.md - documentation index updated

Test plan

• Confirm no other doc or script references the removed inline text by line number
• Confirm the pointers resolve correctly in a fresh session-start read