Feature: Add `ocr delegate task` command to emit a single structured review task

### Problem Statement

`ocr delegate` currently provides the `preview` and `rule` subcommands, which output the "reviewable scope + exclusions" and "review rules grouped by content" respectively. Both are deterministic, LLM-free outputs. Yet they are scattered fragments — there is no command that aggregates everything a host coding agent needs into a single document:

1. **Scattered context.** A host coding agent needs scope, excludes, rules, background, and diffs together to take over a review. Today it must call multiple subcommands and assemble the pieces itself, easily dropping fields.

2. **Missing real diffs.** `preview`/`rule` only describe which files and which rules apply; they do not carry the actual change content. A host agent must obtain the git diff separately, but diff retrieval (commit / range / workspace) is coupled to the internal diff provider and hard to replicate consistently from outside.

3. **Lost fields.** An earlier design summarized the task into Markdown, dropping fields such as `repository`, `merge_base`, the full excluded file paths, and `rules.files`. A host agent reading Markdown saw data not equivalent to the JSON variant, making behavior unverifiable.

### Proposed Solution

Add a `task` subcommand under `ocr delegate` that aggregates all of the above context in one command and emits a single structured review task document:

```bash
# Default Markdown output (human-readable)
ocr delegate task

# Machine-consumable structured JSON
ocr delegate task --format json

# Explicit excludes + background note
ocr delegate task --excludes "**/*_test.go" --background "Payment channel integration; mind concurrency safety"
```

`task` reuses the existing `--format` (`text` = default Markdown / `json`), `--excludes`, and `--background` flags and the `delegateCmd`/`registerDelegateFlags` infrastructure. No new flag design was needed.

### Alternatives Considered

_No response_

### Affected Area

CLI / Commands

### Additional Context

_No response_

Start with the existing `ocr delegate` `preview` and `rule` subcommands, the `delegateCmd` entry point, and `registerDelegateFlags`. Run those commands in text and JSON modes to compare their current outputs; done means `task` combines scope, excludes, rules, background, repository, merge base, and the relevant diff while honoring the existing format and flag behavior.

Written by the indexing model from the issue text.