feat: render plan approval as markdown and capture plans as Todos under Session Workflow - #13445

What

Both parts of #13340 in one draft PR, per the plan discussed with @doudouOUC in that thread.

1. Web Shell: render the ExitPlanMode approval as markdown

packages/web-shell/client/components/messages/ToolApproval.tsx rendered the plan as a raw <pre>{contentText}</pre>, so the markdown the model writes (the exit_plan_mode schema itself declares "Supports markdown") showed up as literal ## / # lines in a fixed-height box.

The isExitPlanApproval branch now renders the plan with the existing Markdown primitive (react-markdown + remark-gfm, with the isSafeHref/isSafeImageSrc sanitisation that matters for model-generated content). The .planContent height clamp is kept, and the raw plan text remains available as the element's title plus the rendered plain text, so the alertdialog's aria-describedby keeps a plain-text accessible equivalent. Non-plan content paths still use <pre>.

2. Core: make the plan-mode reminder ask for a Todo list when Session Workflow is on

getPlanModeSystemReminder() never mentioned todo_write, so with experimental.sessionWorkflow enabled the model almost always wrote a text plan and the structured DAG approval path (planTodos.length > 0) was effectively unreachable (documented in #8393).

• getPlanModeSystemReminder now takes a sessionWorkflow parameter; when it is true (and the interactive exit-plan flow applies, i.e. not plan-only mode) the "When to Converge" section instructs the model to capture the plan as a structured Todo list via todo_write (stable ids, blockedBy for real dependencies) before calling exit_plan_mode.
• All call sites pass the gate through from config: client.ts (per-turn reminder), enterPlanMode.ts (tool llmContent), the ACP session (Session.ts) and nonInteractiveHelpers.ts — all via config.isSessionWorkflowTodoContextActive?.() === true, the same source todo_write already uses for its sessionWorkflow stamp.
• plan-mode-entry-policy.ts now also precomputes the session-workflow variant so the tool-response budget protection keeps matching enter_plan_mode output.

Plan-only mode (SDK/subagent contexts, which never run the interactive exit-plan approval) is unchanged.

Tests

• ToolApproval.test.tsx: new test — the plan renders as markdown (real <h2>/<li>, no raw <pre>, plain-text equivalent preserved for a11y).
• prompts.test.ts: new tests — the session-workflow reminder contains the Todo instruction in the right order; the gate-off and plan-only variants stay text-only; the new variant is exactly the base reminder plus one inserted sentence.
• client.test.ts: updated the call assertion for the new argument.

Local run (vitest, node v26.5.0):

• packages/web-shell: ToolApproval.test.tsx — 81 passed
• packages/core: prompts.test.ts — 205 passed; client.test.ts + coreToolScheduler.test.ts — 956 passed; plan-mode-entry-policy.test.ts + enterPlanMode.test.ts + tool-response-finalizer.test.ts — 59 passed
• eslint + prettier clean on all changed files; tsc --noEmit clean in web-shell and core

Notes

• Draft: both behaviours were agreed in [FEATURE]: Web Shell plan approval: render the plan as markdown and make Plan & Review enforce Todo structure #13340 before this PR, but I'd like a maintainer pass before it becomes ready.
• Related to [FEATURE]: Web Shell plan approval: render the plan as markdown and make Plan & Review enforce Todo structure #13340, feat(web-shell): bind plan approval to its Todo revision #8393.