docs: split agent guide into focused references

[fff-sissimo]

Summary

• keep the root AGENTS.md as a concise 129-line entry point
• move detailed subsystem guidance into focused files under references/agent-guide/
• add explicit read-when routing and a 250-line policy for core instruction documents

Validation

• content preservation check against the previous AGENTS.md: PASS
• relative file links and anchors: PASS
• all core instruction and new reference documents are at most 250 lines
• git diff --check: PASS
• credential-shaped string scan: no findings

Test plan

• Not run (documentation-only change).

[teknium1]

Thanks for the focused routing map and the effort to preserve the existing guide.

Problems

• The split omits the current test-placement policy. Current AGENTS.md:1309-1319 requires assertions about package.json, lockfiles, and JS/TS source to live in the Vitest suite because the CI classifier can skip Python tests on JS-only changes. That policy was added by f8abc521f after this PR's base. The proposed references/agent-guide/testing.md moves from the wrapper explanation at lines 21-33 directly to change-detector guidance at line 36, so the rule would be lost.

Suggested changes

• Transplant AGENTS.md:1309-1319 into references/agent-guide/testing.md, and validate the moved content against current main rather than only the PR base.

Automated hermes-sweeper review.

[teknium1]

Please add the current AGENTS.md:1309-1319 “Where to place what tests” policy before this heading. It was added in f8abc521f after this branch's base and requires JS/package-artifact assertions to live in Vitest; otherwise this split drops an active CI-safety rule.