Docs: CLI reference lists a removed --experimental-zed-integration flag and omits --acp

### What happened?

The flags table in `docs/cli/cli-reference.md` has three problems around ACP, all in two adjacent rows.

**1. It documents a flag that no longer exists.**

```
| `--experimental-zed-integration` | - | boolean | - | Run in Zed editor integration mode. **Experimental feature.** |
```

`experimental-zed-integration` / `experimentalZedIntegration` appears nowhere in `packages/` — the only hit in the whole repo is that documentation line. Passing it does nothing.

**2. It expands the ACP acronym incorrectly.**

The `--experimental-acp` row says "Start in ACP (**Agent Code Pilot**) mode." ACP is the **Agent Client Protocol** — that's what `docs/cli/acp-mode.md` says in its first line ("ACP (Agent Client Protocol) mode"), and it's the name of the open protocol the mode implements.

**3. The current flag is missing, and the deprecated one isn't marked deprecated.**

`packages/cli/src/config/config.ts` defines both:

```ts
.option('acp', {
type: 'boolean',
description: 'Starts the agent in ACP mode',
})
.option('experimental-acp', {
type: 'boolean',
description:
'Starts the agent in ACP mode (deprecated, use --acp instead)',
})
```

So `--acp` is the supported flag and `--experimental-acp` is explicitly deprecated in its own description. The reference table lists only the deprecated one, doesn't say it's deprecated, and never mentions `--acp` at all — even though `docs/cli/acp-mode.md` tells people to run `gemini --acp`.

Net effect: someone reading the reference table is pointed at a deprecated flag and a nonexistent one, and can't discover the flag they should actually use.

### What did you expect to happen?

The table should list `--acp`, mark `--experimental-acp` as deprecated in favour of it (matching how `--yolo` is already handled in the same table), expand ACP as "Agent Client Protocol", and drop `--experimental-zed-integration` since it no longer exists.

### Client information

Found by comparing the docs against the source rather than at runtime, so this isn't version-specific: it's a mismatch between `docs/cli/cli-reference.md` and `packages/cli/src/config/config.ts` on current `main` (package version `0.56.0-nightly.20260806.g761f604c1`).

Reproducible without running the CLI: `grep -rn "experimental-zed-integration" packages/` returns nothing, and `grep -n "\.option('acp'" packages/cli/src/config/config.ts` shows the flag the table omits.

Start with the flags table in docs/cli/cli-reference.md and compare its ACP rows with packages/cli/src/config/config.ts, where the options are defined. Check docs/cli/acp-mode.md for the established ACP wording. Done means the table lists --acp, marks --experimental-acp as deprecated, uses “Agent Client Protocol,” and removes --experimental-zed-integration.

Written by the indexing model from the issue text.