docs: archive trim-agents-handbook change

[Drswith]

Summary

Archive the completed trim-agents-handbook OpenSpec change after PR #71 merged. Sync the accepted AGENTS handbook requirements into openspec/specs/project-memory/spec.md and move the change into openspec/changes/archive/2026-04-28-trim-agents-handbook/.

Linked Artifacts

• Issue: [Docs/Workflow]: Trim AGENTS.md into a thin execution handbook #70 (closed by docs: trim AGENTS handbook #71)
• ADR: docs/adr/0005-keep-agents-as-a-thin-execution-handbook.md
• OpenSpec: openspec/changes/archive/2026-04-28-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 - archive and project-memory sync only

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 already archived in this branch.
• Release is not applicable, delegated to release automation, or verified.

Notes

• bun run test was not run because this follow-up only syncs an accepted spec delta and archives the completed change.
• The archive command initially duplicated the new AGENTS requirements in project-memory; this PR keeps the synced requirement once and removes the duplicate.