Harden autopilot steers against prompt injection; fix scrollback flush flash

[yanmxa]

Two unrelated changes, bundled at author request (see the two commits).

Autopilot steer hardening

Guards the autopilot LLM steers (Permission, Bash, Question, turn-end
continue) against prompt injection from untrusted content:

• Composes each steer's system prompt as an immutable control-plane policy
  plus the user's steering instructions — policy in the recency slot, forged
  </steering_instructions> delimiters stripped — so a shared preset or
  systemPromptFile can't weaken the trust boundaries.
• Routes all untrusted payloads (tool args, transcripts, questions, bash
  prompts) through one JSON data envelope (reviewer.RenderDataEnvelope);
  json.Marshal escapes <, >, &, so values can't forge the delimiters.
• Sanitizes bash-prompt answers to a single bounded line and validates the
  continue/done/stop decision as a strict state machine.
• Renames the user-facing "System Prompt" to "Steering Prompt".

Scrollback flush fix

Removes the blank-frame flash at the end of a streamed render by keeping a
committed block visible in the managed view until its tea.Println into
native scrollback has been processed; monotonic ids keep concurrent handoffs
ordered.

[san-ci]

[PR TITLE LINT] PR title does not follow Conventional Commits format.

Please update your PR title to follow Conventional Commits format:

type(scope): description

Allowed types:
feat | fix | docs | style | refactor | perf | test | build | ci | chore | revert

Examples:

feat: add user login
fix(api): handle null response
docs: update readme
refactor: extract auth middleware

You can edit the title at the top of this PR.

[san-ci]

[REVIEW NOTIFIER] This PR requires a review from @ldpliu.

Once this PR has been reviewed and has the lgtm label, it will be automatically squash-merged.

Suggested reviewer

Based on OWNERS, @ldpliu is suggested to review this PR.

How to get your PR reviewed:

• Address reviewer feedback and push changes
• Reviewer adds lgtm label when satisfied
• An approving review + lgtm will trigger auto-merge

[san-ci]

[REBASE NOTIFIER] Hi @yanmxa! This PR has merge conflicts.

The lgtm label has been removed and the reviewer will need to re-review after you fix this.

How to fix this

resolve the conflicts — Fix the merge conflicts locally, then push the resolved changes.

After fixing:

• The need-rebase label will be automatically removed
• Request a new review from the OWNERS file
• The reviewer can add lgtm once satisfied