[Feature]: Restore the native context strategy option in the web UI (scroll prompt is too heavy)

## Summary

v2.1.0 removed the context-strategy selector (scroll / native) from the console, so users are locked into `scroll`. The backend still fully supports `native` (`LightContextConfig.strategy: Literal["native", "scroll"]`), but there is no longer any UI — or documented — way to choose it. Please restore the `native` option, or, if it is being deprecated, provide a lighter-weight scroll prompt.

## Component(s) Affected

- [x] Core / Backend (app, agents, config, providers, utils, local_models)
- [x] Console (frontend web UI)
- [ ] Channels (DingTalk, Feishu, QQ, Discord, iMessage, etc.)
- [ ] Skills
- [ ] CLI
- [ ] Documentation (website)
- [ ] Tests
- [ ] CI/CD
- [ ] Scripts / Deploy

## Problem / Motivation

1. **`native` is still useful.** It is the lightweight strategy (AgentScope's built-in compression) with no extra system prompt. For single-session tasks, or users who do not need durable cross-session recall, `native` is simpler, faster, and more predictable.

2. **`scroll` injects a very large system prompt.** `src/qwenpaw/agents/context/scroll/prompt.py` (`SCROLL_SYSTEM_PROMPT` / `SCROLL_SYSTEM_PROMPT_ZH`) is ~96 lines / ~930 words, covering the `⟦ headline ⟧` format, the `[context compressed]` map, `recall_history` usage, and a long DISCIPLINE block. This consumes a meaningful share of the context budget and, more importantly, adds heavy instruction-following overhead that can distract or over-constrain the model — especially for smaller/faster models or simple tasks that do not benefit from it.

3. After commit `734c8b9f` (#6323) removed the selector from `console/src/pages/Agent/Config/components/ReactAgentCard.tsx`, there is no in-UI opt-out: every agent pays the scroll overhead by default.

## Proposed Solution

**Option A (preferred):** restore the `light_context_config.strategy` selector (scroll / native) in the console, e.g. in `ReactAgentCard` or `LightContextCard`.

**Option B:** if `native` is on a deprecation path, keep the selector but clearly mark `native` as legacy and document a migration path.

**Option C (minimum):** keep the strategy configurable and document `light_context_config.strategy` in `config.json` (it already works, but is hidden and requires a restart), and/or trim the scroll system prompt into a small mandatory core plus optional detail.

## Alternatives Considered

- Manually setting `light_context_config.strategy = "native"` in `config.json` works today, but requires a restart and is undocumented — not discoverable for most users.

## Additional Context

- QwenPaw version: 2.1.0 (the removal landed between 2.0.1 and 2.1.0).
- Removing commit: `734c8b9f` — "feat(scroll): add staged compaction and durable task continuity (#6323)".
- The i18n keys `agentConfig.contextStrategyScroll` / `agentConfig.contextStrategyNative` still exist in the locale files but are now orphaned (no component renders them), which suggests the selector was removed without fully deprecating `native`.

## Willing to Contribute

- [ ] I am willing to open a PR for this feature (after discussion).