fix(tui): avoid stacking editor layers on session start

[outlier27-cell]

Problem

Repeated Pi session_start events on one TUI context rebuilt OpenPI's editor factory from the already-wrapped factory. Each /new, /resume, or /reload could therefore stack every registered editor layer again.

Closes #740

Value

The active editor keeps one composition of each OpenPI layer across session starts, preventing duplicated editor behavior and nested rendering overhead.

Approach

• Capture the base editor factory when the coordinator first receives an active TUI context.
• Compose registered layers from that preserved base until the context is released.
• Add a regression that registers the same layer twice on one context and checks the wrapper depth remains one.

Validation

• node --test --experimental-strip-types tests/extensions/shared/editor-layers.test.ts (2 passing)
• bun run check
• bun run test reproduced existing failures in unmodified clear-context, renderer-chain, and Git fixture suites; the new editor-layer regression passed before the run was stopped. Exact-head GitHub CI is tracked for the final cross-platform result.

Impact

No new UI surface or configuration. Editor layers retain their existing registration order and lifecycle while repeated session starts remain idempotent.

[outlier27-cell]

@tt-a1i 请审核此 PR。当前精确提交的 GitHub CI 已全部通过，修改范围与回归测试均已在 PR 说明中列明；现仅等待维护者审核。