Add skill to update SDK dependencies

Also adds an owners file for skills, allowing all eng.

Change-Id: Iefc970e31d3c2168b5b66e05b413e569072013be
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/493560
Reviewed-by: Reid Baker <reidbaker@google.com>
Auto-Submit: Jake Macdonald <jakemac@google.com>
Commit-Queue: Jake Macdonald <jakemac@google.com>
Reviewed-by: Alexander Thomas <athom@google.com>

.agents/skills/OWNERS

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+# Allow all committers to review documentation CLs.
+file:/tools/OWNERS_ENG

.agents/skills/update_dependency/SKILL.md

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+---
+name: update-dependency
+description: Updates third party dependencies in the Dart SDK
+---
+
+# Instructions
+
+Use this skill to update dependencies in the Dart SDK's `DEPS` file to their
+latest (or a specific) version. This tool automates the process of updating
+the version, syncing dependencies, updating package configurations, and
+creating a CL.
+
+## Prerequisites
+
+Before running the tool, ensure your environment is prepared correctly:
+
+1.  **Clean Git State**: Your git checkout MUST be clean. No uncommitted
+    changes should be present.
+2.  **On Main Branch**: You MUST be on the `main` branch. The tool creates a
+    new branch from your current HEAD, so being on `main` ensures the bump
+    starts from a clean baseline.
+
+## Tool Usage
+
+Run the `tools/manage_deps.dart` script with the `bump` command from the root
+of the SDK.
+
+### Command Syntax
+
+```bash
+dart tools/manage_deps.dart bump <path/to/dependency> \
+  [--branch <branch_name>] [--target <ref>]
+```
+
+### Arguments
+
+1.  `<path/to/dependency>` (required): The path to the dependency directory
+    relative to the SDK root. Dependencies are entire repositories and not
+    packages, so the path will usually be `third_party/pkg/<repo>`.
+    - **CRITICAL**: DO NOT include a trailing slash in the path.
+    - If unsure of the repository, you can look at the
+      `.dart_tool/package_config.json` file relative to the SDK root, and search
+      for `"name": "<package-name>"`. Next to that you should see a "rootUri"
+      key which will give you the path to the dependency. Do not use the full
+      path though, just the top level directory under `third_party/pkg/<repo>`,
+      since entire repos are always rolled at once.
+    - If the `.dart_tool/package_config.json` file does not exist, it can be
+      generated by running `gclient sync`.
+2.  `--branch <branch_name>` (optional): The name of the branch to create for
+    this update. Defaults to `bump_<dependency_name>`.
+3.  `--target <ref>` (optional): The specific git ref (SHA, tag, branch) to
+    update to. Defaults to the latest version on the remote's default branch.
+
+## Example
+
+To bump the `unified_analytics` dependency, which is a part of the `tools` repo:
+
+```bash
+dart tools/manage_deps.dart bump third_party/pkg/tools
+```
+
+## Troubleshooting
+
+- **"Already at <target> - nothing to do"**: The dependency is already at the
+  latest version or the specified target.
+- **Branch already exists**: If the tool says a branch with the target name
+  already exists, you will be prompted to delete it. Usually, it is safe to say
+  "y" if you want a fresh bump.