Add git commit skill for AI agents

Include the .agents/skills/commit/SKILL.md file which provides a skill
for AI agents to make commits in the project following the repository's
standards.

Also update deno.json to exclude the .agents/skills/ and .claude/skills/
directories from Deno's processes. These directories contain markdown
and other configuration files for AI agents that do not need to be
processed by Deno.

[ci skip]

Author: dahlia

.agents/skills/commit/SKILL.md

@@ -0,0 +1,60 @@
+---
+name: commit
+description: Make a commit in Git with an appropriate commit message for the currently staged changes.
+allowed-tools: Bash(git *)
+---
+
+Please make a commit with an appropriate commit message for the currently
+staged changes.
+
+Don't touch the staged changes, just create the commit.
+
+The first line of the commit message should be a concise summary of the
+changes (50 characters or less).  You shouldn't follow the conventional
+commits, in other words, just write a normal commit message without any
+special prefixes.
+
+The second line of the commit message should be blank.
+
+The rest of the commit message should provide more detailed explanations
+about the changes, wrapped at 72 characters.
+
+If you have some references like issues or pull requests, you can add
+them in the body of the commit message, usually after a blank line.
+Issues and pull requests should be referenced as full URLs instead of
+#numbers, and you can put some verbs like “Fixes” or “Closes” before
+the URLs.  For just references without any verbs, you can just put the
+URLs in the body.
+
+When running `git commit`, pass the commit message via a HEREDOC to
+preserve newlines correctly.  Never use literal `\n` escape sequences
+inside a quoted string, as they will appear as literal characters in
+the commit message.  For example:
+
+~~~~ bash
+git commit -m "$(cat <<'EOF'
+Summary line here
+
+Detailed explanation here.
+EOF
+)"
+~~~~
+
+Don't put `Generated with [Claude Code](https://claude.ai/code)`, or
+`Co-authored-by` trailers at the end of the commit message.  Instead,
+use the `Assiged-by` trailer to indicate that the commit was generated by
+an AI assistant, if necessary.  The format of the `Assiged-by` trailer should
+be:
+
+~~~~
+Assisted-by: AGENT_NAME:MODEL_VERSION
+~~~~
+
+For example:
+
+~~~~
+Assisted-by: OpenCode:qwen3.6-plus
+Assisted-by: Claude Code:claude-sonnet-4-6
+Assisted-by: Gemini CLI:gemini-3.1-pro-preview
+Assisted-by: Codex:gpt-5.4
+~~~~