docs: trim AGENTS handbook

[Drswith]

Summary

Closes #70.

Rewrite AGENTS.md into a thin execution handbook that keeps hard workflow constraints inline and routes drift-prone details through trigger-based pointers. Promote discussion 68 into issue, session, ADR, and OpenSpec artifacts so the rationale and contract live in the repository.

Linked Artifacts

• Issue: [Docs/Workflow]: Trim AGENTS.md into a thin execution handbook #70
• ADR: docs/adr/0005-keep-agents-as-a-thin-execution-handbook.md
• OpenSpec: openspec/changes/trim-agents-handbook/
• Discussion: AGENTS.md should be thin handbook + trigger-based pointers #68

Validation

• bun run memory:check
• bun run lint
• bun run typecheck
• bun run test (if behavior changed)
• Not run, explained below

Release Intent

• Release: not applicable - docs/process-only handbook and project-memory change

Docs Updated

• Not needed
• docs/...
• openspec/...
• Follow-up issue or OpenSpec change created instead

Scope Check

• I did not add a new ad hoc root-level Markdown file.
• I updated the relevant issue, ADR, spec, runbook, or captured the missing doc work as follow-up.
• I did not silently expand project scope without recording it explicitly.

Closure Check

• Working tree was clean after commit.
• Branch was pushed and this PR is the active delivery artifact.
• OpenSpec change is still active by design until merge.
• Release is not applicable, delegated to release automation, or verified.

Notes

• bun run test was not run because this change does not alter runtime CLI behavior.
• OpenSpec archive closure remains post-merge work; the change should be archived after the accepted spec delta is synced on the protected branch.