[camera_android_camerax] Add presubmit sweep, cross-platform paths, and test diff guidelines to AGENTS.md (#12668)

These suggestions came from /learn. No evals because the infra required
to test the actually valuable one requires interactions with remote
services. I am going to brainstorm on how we can/should evaluate remote
interactions.

http://github.com/flutter/packages/pull/12681 is a pr that will not
required the override labels for prs like this.

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Agent authored description

## Description

Adds key agent guidelines to `camera_android_camerax/AGENTS.md`:
- **Presubmit Sweep Fixes**: Directs agents to wait until all builder
targets in the presubmit matrix complete before pushing, reproducing and
fixing all failures locally to push in one consolidated commit.
- **Cross-Platform Paths in Tests**: Enforces constructing CLI arguments
and file paths with `package:path` (`p.join` / `p.joinAll`) rather than
hardcoded `/` to avoid test failures on Windows CI bots.
- **Preserve Test Order**: Directs agents to maintain the original
sequence of existing unit tests when adding or updating tests to
minimize review diffs.

## Tests
- Validated `flutter test test/skills_evals_test.dart
test/validate_skills_test.dart` (all passing).

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Co-authored-by: Reid Baker <reidbaker@google.com>