feat: configure Herdr refresh intervals

[leesangb]

Summary

• add configurable focused, background, and pull request refresh intervals under herdr.refresh
• merge partial shared and local Herdr refresh settings without dropping sibling values
• clamp configured intervals to safe minimums
• re-read settings while watchers run and preserve immediate refresh on Space focus
• derive metadata TTLs from the configured refresh periods

Defaults

• focusedSeconds: 30, minimum 5
• backgroundSeconds: 300, minimum 30
• pullRequestSeconds: 300, minimum 60

Validation

• bun test
• bun run typecheck
• bun run build:herdr-plugin
• git diff --check

[chatgpt-codex-connector]

Codex usage limits have been reached for code reviews. Please check with the admins of this repo to increase the limits by adding credits.
Credits must be used to enable repository wide code reviews.