Centralize constants in one file

[github-actions]

Move default trigger phrases (e.g., /codex) and configuration values (e.g., progress-bar block counts) into a single constants module.

[YiweiShen]

/codex Move default trigger phrases (e.g., /codex) and configuration values (e.g., progress-bar block counts) into a single constants module.

[YiweiShen]

/codex Move default trigger phrases (e.g., /codex) and configuration values (e.g., progress-bar block counts) into a single constants module.

[github-actions]

Created Pull Request: #399