feat(planning): move the type rules into a shared type-rules convention read by design, implement and the reviewer #12150

Parent #7589 decides that pstack's principles are placed in the phase skills that apply them (D58). Decision 58, approved by the owner (poteto-review decision row 58), resolves the one part with no home: part 24j, "apply the same type rules while writing code". It reads: "Type rules move to a docs/conventions type-rules convention synced into plugins." The reason: the rules live in plugins/planning/skills/design/reference/type-discipline.md, which only /planning:design may read. The skill-encapsulation rule bars /implementation:implement and review:code-reviewer from citing it, and the reviewer restates part of it instead. The precedent named in the decision is the enforcement ladder, which now lives at docs/contracts/enforcement-ladder/README.md (docs/conventions holds plugin settings files per ADR 0064), as a contract plus byte-identical plugin copies registered in scripts/shared-copies.txt and generated by scripts/sync-shared-copies.sh. On main, plugins/planning/skills/design/reference/type-discipline.md is read by design's SKILL.md Phase 3, links to type-discipline/typescript.md (which links back to ../type-discipline.md), and is asserted by two cases in plugins/planning/skills/design/evals/evals.json.

Done when

• docs/contracts/type-discipline/README.md holds the canonical rules table moved from plugins/planning/skills/design/reference/type-discipline.md, with every rule, its decision, its reason and their order unchanged. It carries no wording specific to /planning:design (Phase 3, type-inventory.md) and no relative link that would not resolve from every copy's location.
• scripts/shared-copies.txt lists a copy for each of plugins/planning, plugins/implementation and plugins/review. The planning copy is generated at the existing path plugins/planning/skills/design/reference/type-discipline.md, so design's SKILL.md link, type-discipline/typescript.md's link back, and the two evals.json cases that name that path keep resolving. scripts/sync-shared-copies.sh generates the copies, and CI's copy check passes.
• /planning:design's SKILL.md keeps the Phase 3 framing and the language-file pointer (TypeScript: type-discipline/typescript.md, loaded when a tsconfig.json or TypeScript files are present) that leave the shared text.
• /implementation:implement reads its copy at the step where it writes types, with one reasoned sentence on when it applies.
• review:code-reviewer reads its copy for type questions. Its existing items, the cast item included, are unchanged.
• .changes/planning/*.md, .changes/implementation/*.md and .changes/review/*.md fragments describe the change (bump: patch).
• The PR's CI is green.

Out of scope

• Adding new type rules or changing existing ones.
• Removing the reviewer's own type-rule restatements in favor of the copy: cutting existing agent text needs eval evidence of no loss first (skill criteria).
• Gated part 24i (optional-field bags in review).

Parent: #7589