feat: --show-gitignored flag for file picker (@-syntax) to show gitignored files

[steilerDev]

Problem

The @ file picker respects .gitignore, hiding gitignored files from the UI. For agents with a local knowledge base that is gitignored (e.g., because it contains account data or runtime content that shouldn't be versioned), this creates a painful UX gap:

• The agent can read the files — filesystem tools have no issue accessing them
• But pointing the agent at a specific file via @ is cumbersome; users must type full paths manually

The .gitignore contract is "don't version-control this" — not "hide from local tools." For an agent operating on the local filesystem, these are orthogonal concerns.

Proposed Solution

Add a --show-gitignored runtime flag to docker agent that controls whether the file picker respects .gitignore:

docker agent run --show-gitignored tampa.yaml

This mirrors the existing ignore_vcs option on filesystem read tools but exposed at the CLI level — no config file changes needed.

Optionally, support an .agentignore file (same syntax as .gitignore) so users can explicitly hide files from the picker without coupling that decision to VCS:

# .agentignore
secrets/
*.key

This keeps agent-visibility concerns separate from VCS concerns — users opt in to showing gitignored content at startup, and opt out of specific paths via .agentignore if needed.

[joshbarrington]

The snacks file picker in nvim (https://github.com/folke/snacks.nvim/blob/main/docs/picker.md) provides hot keys Alt-i and Alt-h in the picker view to toggle both hidden and ignored files in the available list/search options.

An implementation such as this would be an even better UX, as it doesn't require the user to launch the agent with specific flags, you can simply toggle the available files once in the picker view. Thoughts?