ops(guardrails): enforce checkpoint commits during execution

[c-wri]

Summary

• tighten repo-local standards, agent routes, templates, and guard tests so checkpoint commits are planned up front and created during execution rather than reconstructed at task end

Problem

• the repo already expects stable checkpoint commits, but the strongest wording is concentrated in closeout surfaces such as docs/standards/implementation.md, docs/standards/commits.md, and .claude/commands/implementation-checkpoint.md
• task-start surfaces are inconsistent, so agents can still treat commits as end-of-task cleanup instead of part of execution
• current repo-native automation validates commit message shape and PR checkpoint metadata after commits already exist, but does not pin the checkpoint-timing rule across standards, skills, routes, and templates

Evidence

• AGENTS.md says to commit at stable checkpoints without waiting to be reminded, but it does not currently serve as the canonical route for planning checkpoint slices before substantial edits
• docs/standards/implementation.md and .claude/commands/implementation-checkpoint.md emphasize committing before closing, which is correct but still leaves room for retroactive end-of-task slice reconstruction
• .agents/skills/implementation-workflow/SKILL.md, .agents/skills/adapter-authoring/SKILL.md, and .agents/skills/reconciliation-tax-build/SKILL.md do not consistently force checkpoint planning and in-flight commit creation at task start
• tests/unit/test_repo_agent_skills.py, tests/unit/test_docs_runtime_parity.py, and tests/contract/test_standards_guards.py do not currently pin this rule tightly enough to catch future control-plane drift

Desired Outcome

• add one canonical repo-local checkpoint-discipline route under .claude/commands/ and reference it from the owning standards and task-start skills
• align repo-local skills, command routes, commit template, and PR template around the same invariant: define checkpoint slices before substantial edits and commit each stable slice before moving on
• add repo-native guard tests so this rule cannot silently weaken again

Acceptance Criteria

• a new checkpoint-discipline route exists and is referenced from AGENTS.md, docs/standards/implementation.md, docs/standards/commits.md, docs/standards/delivery-guardrails.md, and .claude/commands/implementation-checkpoint.md
• repo-local implementation-oriented skills and command routes require checkpoint planning and in-flight commit creation rather than only final closeout commits
• .gitmessage.txt and .github/pull_request_template.md reinforce checkpoint timing without adding broad workflow noise
• control-plane tests pin the rule across standards, skills, routes, and templates
• the repair lands as multiple stable checkpoint commits on its implementation branch rather than one late umbrella commit