refactor(migrate): rename DSL _kind 'checkpoint' → 'sidebar'

[icarthick]

Summary

Pure concept rename, no behavior change: the DSL phase kind _kind: checkpoint becomes _kind: sidebar.

Why. "checkpoint" misleadingly implies a mandatory inline gate you pass through (its meaning in git/CI/DBs). The construct is the opposite: an optional phase that hangs OFF the backbone — entered only by its _trigger, returns control instead of advancing, never becomes current_phase. "sidebar" captures that step-off-and-return nature, and reads cleanly against _kind: backbone. It also matches the "side-visit" language the interpreter contract already uses to explain the construct.

Changes (46 files, +239/−239 — symmetric, pure rename)

• Validator (tools/frontmatter-validator/{parse,types,check}.ts): the _kind role value and all error-message strings checkpoint → sidebar. _kind: checkpoint without a parser change would silently parse as backbone and fail — so this is a lockstep grammar-vocabulary change. Test suite (frontmatter-validator.test.ts) updated — 57 tests pass.
• All 18 _kind: checkpoint frontmatter declarations → sidebar across every skill (heroku, vercel, gcp, agent-advisor, shared).
• Construct prose checkpoint → sidebar in canonical INTERPRETER.md, workshop-invariants.md, phase-status.schema.json (doc string), and per-skill files. Vendored copies re-synced byte-identical (shared:check OK across 4 trees).

Protected — NOT renamed (external/unrelated terms)

Deliberately skipped 5 files whose "checkpoint" is not the DSL construct:

• LangGraph checkpointer — retarget-gotchas.md, clarify-ai.md
• Temporal "checkpoint business state" — decision-refs/temporal.md
• "Go/No-Go checkpoint" PM milestones — migration-complexity.md, generate-infra.md

Deferred to follow-ups (intentionally out of scope)workshop→what-ifphase rename (phase id, dirs,phases.workshop` state key, fixtures)

• estimation_summary.workshop metadata field rename
• Tier 2 — structural _gates: make the sidebar's "hold the backbone successor until it resolves" declarative + validator-enforced (today it lives in interpreter prose). That's where the real capability win is; this PR is vocabulary only.

Test plan

• mise run build green — frontmatter validator (all skills), node --test (57 pass), dprint fmt:check, markdownlint, shared:check (4 vendored trees byte-identical), security scanners.

Note for reviewers

This is a plugin-wide vocabulary change, so it touches **/SKILL.md (→ @startups-admins) and the agent-advisor / llm-to-bedrock skills. The diff is mechanical and symmetric (+239/−239); no logic changes.