feat: persist Hunk view preferences

[benvinegar]

Summary

• add layered Hunk config loading with global and repo-local TOML files
• persist theme/layout/view toggles between invocations, including pager mode
• add CLI overrides for persistent view prefs and document the new config model

Details

• support $XDG_CONFIG_HOME/hunk/config.toml and ~/.config/hunk/config.toml
• support repo-local .hunk/config.toml
• apply precedence: built-ins < global config < repo config < command/pager profiles < CLI flags
• persist view prefs back to .hunk/config.toml in repos, or global config outside repos
• add command/pager profile sections like [pager], [git], [diff], [patch], [difftool]
• add CLI flags for line numbers, wrapping, hunk headers, and agent notes

Validation

• bun run typecheck
• bun test
• bun run test:tty-smoke

[sentry]

Bug: The system-detected theme is incorrectly persisted on first launch, which prevents the application from automatically following system theme changes on subsequent runs.
_{Severity: HIGH}

Suggested Fix

Modify the persistence logic to only save the theme to the configuration file if the user has explicitly selected one. This can be achieved by tracking whether the theme was user-selected or inferred from the system default, and only persisting it in the former case. This will allow the application to re-detect the system theme on each launch if no explicit theme is set.

Prompt for AI Agent

Review the code at the location below. A potential bug has been identified by an AI
agent.
Verify if this is a real issue. If it is, propose a fix; if not, explain why it's not
valid.

Location: src/core/config.ts#L274-L278

Potential issue: On the first run, the application detects the user's system theme
(e.g., 'dark' mode resolves to the 'midnight' theme) and immediately persists this theme
to the configuration file. The persistence logic in `src/core/config.ts` always saves
the `preferences.theme` value because the `resolveTheme` function ensures it is never
undefined. As a result, on all subsequent launches, the application uses this saved
theme instead of re-evaluating the current system theme. This prevents the application
from automatically adapting to system theme changes after its initial execution, locking
it into the theme from the first run.