feat: add shell completion for bash and zsh

[mifisignal]

Add agent-browser completion <shell> command that prints a shell completion script for bash or zsh. Introduce named module-level constants for every subcommand list (replacing local const blocks) so the new completion test suite can assert that each script covers all top-level commands, subcommands, and global flags.

[vercel]

@mfischer-zd is attempting to deploy a commit to the Vercel Labs Team on Vercel.

A member of the Team first needs to authorize it.

[ctate]

Great work — the completion scripts are thorough and the test macro that cross-checks against Rust source is a smart way to prevent drift. Three things needed before merging:

1. Run cargo fmt — several long const arrays in commands.rs exceed the line width limit, so CI will fail without this.
2. Add docs to skills/agent-browser/SKILL.md and docs/src/app/ per project convention (AGENTS.md requires all 4 doc locations updated for new user-facing commands; output.rs and README.md are already done).
3. Typo — completion.rs:117: "autoompletion" → "autocompletion".

[mifisignal]

Great work — the completion scripts are thorough and the test macro that cross-checks against Rust source is a smart way to prevent drift. Three things needed before merging:

1. Run cargo fmt — several long const arrays in commands.rs exceed the line width limit, so CI will fail without this.

✅ done

2. Add docs to skills/agent-browser/SKILL.md and docs/src/app/ per project convention (AGENTS.md requires all 4 doc locations updated for new user-facing commands; output.rs and README.md are already done).

✅ done, with one exception: The autocomplete functionality is to assist users who are using the tool interactively in a shell, so I can't imagine a situation in which a coding agent would bother with it. In light of that, do you agree that we can skip updating the Skill?

3. Typo — completion.rs:117: "autoompletion" → "autocompletion".

✏️ fixed

Let me know if there's anything else @ctate !