feat(provider): forward sessionOptions (systemPrompt, model, allowedTools, maxTurns) to runtime

[DaniAkash]

⚠️ Draft — waiting on upstream

This PR is pre-implementation against the documented upstream shape. It will not pass typecheck until openclaw/acpx#309 merges and a corresponding acpx release ships (expected as 0.8.0 based on the documented type signatures, matching the version assumed by #17 and #19).

Do not mark ready / merge until:

1. feat(runtime): accept sessionOptions on ensureSession openclaw/acpx#309 is merged.
2. acpx@0.8.0 (or whatever version lands the sessionOptions field) is published to npm.
3. Locally: bun install resolves the new acpx, then bun run typecheck passes — confirming upstream's published types match the documented shape.
4. Verified the published runtime.d.ts exposes sessionOptions?: SessionAgentOptions on AcpRuntimeEnsureInput plus the SessionAgentOptions + SystemPromptOption re-exports. If upstream renamed during review, update src/types.ts, src/provider.ts, and src/index.ts to match.

Summary

Hosts can now set a per-session systemPrompt (or model / allowedTools / maxTurns) on a fresh ACP session through the provider — no need to bypass the runtime.

```ts
const provider = createAcpxProvider({
agent: 'claude',
sessionOptions: {
systemPrompt: 'You are an expert Rust reviewer. Be terse.',
// model: 'claude-opus-4-7',
// allowedTools: ['read', 'edit'],
// maxTurns: 5,
},
})
```

`{ append: '…' }` is also supported when you want to extend the agent's default prompt instead of replacing it. System prompts are fixed at `session/new` time — to switch prompts for the same workspace, use a distinct `sessionKey` or close the prior session first. Reused persistent records ignore `sessionOptions` by design.

This unblocks AI-SDK consumers that want per-conversation system prompts (chat apps, evaluation harnesses, scoped agents). The matching runtime change at openclaw/acpx#309 opens `AcpRuntime.ensureSession` to accept `sessionOptions`; this PR exposes that on `AcpxProviderSettings` and threads it through `ensureHandle`.

Files changed

File	Change
`src/types.ts`	Re-export `SessionAgentOptions` + `SystemPromptOption` from `acpx/runtime`; add `sessionOptions?: SessionAgentOptions` to `AcpxProviderSettings` with the inline doc explaining the reuse-vs-fresh semantics
`src/provider.ts`	Thread `this.settings.sessionOptions` from `ensureHandle` into `runtime.ensureSession(...)` (one line)
`src/index.ts`	Re-export the two new types
`src/mcp-servers.ts`	Drive-by: `export` the `RuntimeMcpServer` type so fallow's "private type leak" check passes (a regression introduced in #22 — fallow currently fails on main)
`README.md`	New `### System prompts and per-session agent options` subsection under `## Persistent sessions`, covering string + `{ append }` forms, the fresh-vs-reused semantics, and the cross-agent caveat
`test/unit/session-options.test.ts`	NEW — three tests: `systemPrompt` string forwards through, `{ append }` + `model` + `allowedTools` + `maxTurns` round-trip, omitting `sessionOptions` keeps the runtime call clean
`package.json`	Version `0.0.3` → `0.1.0`; `acpx` peer-dep + dev-dep `>=0.6.1` → `>=0.8.0`

Test plan

Check	Status	Why
`bun run typecheck`	❌ 6 errors	All from missing upstream types (`sessionOptions` on `AcpRuntimeEnsureInput`, `SessionAgentOptions` / `SystemPromptOption` from `acpx/runtime`) — expected, will go green when `acpx@0.8.0` publishes
`bun run lint`	✅ clean
`bun run fallow`	✅ exit 0	Drive-by `export` of `RuntimeMcpServer` was required to get this green
`bun test`	✅ 198 pass / 15 skip / 0 fail / 300 expect()	TS types are erased at runtime; the 3 new tests are part of the 198
`bun run build`	✅ ESM + .d.ts (~32 KB)	bunup's esbuild path strips types without re-checking

Coordination with #17 and #19

Three drafts now target the same upstream `acpx@0.8.0` release: #17, #19, and this PR. All three bump `version` to `0.1.0` and `acpx` to `>=0.8.0`. Whichever lands second and third needs to bump its version one step further (`0.1.0` → `0.2.0` → `0.3.0`, etc.) and resolve the `package.json` conflict on `version` + the dep range.

Known issue: dev-dep won't resolve from a clean install

The dev-dep range `acpx >=0.8.0` doesn't have a matching version published yet. `bun install` from a fresh checkout of this branch will fail. This is expected for the draft phase. Branch exists for review, not for fresh installs.

When upstream lands, the resolver will pick up `acpx@0.8.0` automatically and everything goes green.