feat(plan): run the plan-review gate on every plan-mode entry path

[mbuckaway]

Related Issue

No upstream issue — fork-only prompt change (tracked in fork/PATCHES.md).

Problem

The plan-review gate shipped in 0.36.0-MB.1.20 lives only in the EnterPlanMode tool-result message. Sessions entered via /plan, Shift-Tab, the --plan flag, default_plan_mode, or a resume never receive that message, so the gate never fires for them — plans produced on those entry paths go un-reviewed.

What changed

Prompt text in both engines (agent-core v1 and agent-core-v2); no tool-code change, no system-prompt change, EnterPlanMode message unchanged:

• The plan-review gate is added to every plan-mode reminder variant (full / sparse / reentry + inline twins), using the exact wording already shipped in the EnterPlanMode message: before ExitPlanMode, check the plan-review skill is in the skill listing, run it once when available and the plan file lacks a ## Review Findings section, resolve every finding, and let the skill's close-out call ExitPlanMode (never call it twice).
• Full and inline-full reminders gain the gate as a workflow step before Exit (renumbered); sparse/inline-sparse append the gate sentence; reentry/inline-reentry gain a gate step.
• Tests updated for the new reminder text; token-count inline snapshots regenerated.

Checklist

• I have read the CONTRIBUTING document.
• I have linked a related issue, or explained the problem above.
• I have added tests that prove my feature works.
• Ran gen-changesets skill, or this PR needs no changeset. (changeset added: .changeset/plan-gate-reminders.md)
• Ran gen-docs skill, or this PR needs no doc update. (prompt behavior only; docs describe the tool, not the reminders)