feat: add config option for sidebar default state

[marcelarie]

Adds a new tui.sidebar config option to set the sidebar's initial state. Accepts "auto" (default), "show", or "hide".

• Config provides initial default before user interaction
• User toggles via keybind are stored in KV and take precedence
• Priority: KV store > config.tui.sidebar > "auto"

Closes #3682

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

Summary

No duplicate PRs found. The search returned the PR itself (6955) but no other open PRs addressing the same feature.

However, I found related PRs that might be worth reviewing for context:

1. PR #6594 - feat(tui): add per-workspace TUI display settings

   • Related because it also deals with TUI configuration/display settings

2. PR #6092 - feat: add config option to disable sidebar overlay mode

   • Related as it's another sidebar configuration feature

3. PR #6093 - feat: add persistent sidebar overlay toggle in command palette

   • Related as it handles sidebar state persistence

These are not duplicates but may have overlapping concerns around sidebar state management and persistence that could benefit from coordination.

[marcelarie]

The following comment was made by an LLM, it may be inaccurate:
Summary

No duplicate PRs found. The search returned the PR itself (6955) but no other open PRs addressing the same feature.

Changes After Reviewing Related PRs

After reviewing #6594, #6092, and #6093,the following issues were identified and fixed:

1. Priority order: Changed from KV > Config to Config > KV to enable per-workspace settings (matching feat(tui): add per-workspace TUI display settings #6594's approach)
2. Schema definition: Changed from .default("auto") to .optional() to allow KV fallback when config is unset

[github-actions]

Hey! Your PR title feat(tui): add config option for sidebar default state doesn't follow conventional commit format.

Please update it to start with one of:

• feat: new feature
• fix: bug fix
• docs: documentation changes
• chore: maintenance tasks
• refactor: code refactoring
• test: adding or updating tests

See CONTRIBUTING.md for details.