docs: index agent guidance sources

[rianjs]

Summary

• add AGENTS.md as a peer Codex entrypoint
• shrink CLAUDE.md into a thin source index
• move Slack-specific development facts into docs/development.md while linking shared standards and automation to their canonical repos

Verification

• git diff --check
• rg -n -F "Source of truth:" AGENTS.md CLAUDE.md docs/development.md
• rg -n -F "Local convenience copy, if present:" AGENTS.md CLAUDE.md docs/development.md
• rg peer-entrypoint checks for AGENTS/CLAUDE cross-dependencies
• rg duplication guard for shared CI/release mechanics
• make test
• make lint
• make build

Closes #181

[rianjs]

Major
docs/development.md carries forward inaccurate repo-local facts into the new canonical guide. make test is described as running “race detection and coverage,” but the Makefile’s test target runs go test -v -race ./...; coverage is under test-cover. The repo structure also lists root main.go, but the binary entrypoint is cmd/slck/main.go and the Makefile builds ./cmd/slck. Fix both before merging so the repo-local guide is trustworthy.

[rianjs]

Major
docs/development.md still has one stale quick-command fact: make install is documented as installing to $GOPATH/bin, but the Makefile’s install target runs install -m 755 bin/$(BINARY) /usr/local/bin/. Update that comment so the new repo-local guide reflects the actual install destination.

[rianjs]

No findings.

The updated diff resolves the local factual issues. The PR now matches the initiative pattern: peer thin entrypoints, Slack-specific facts in docs/development.md, canonical source/local references for shared standards and automation, and no duplicated CI/release mechanics in the touched guidance.