Adds read-only init dry-run planning.

Scope:

• add init --dry-run command;
• add read-only init plan model;
• plan AGENTS.md creation when missing;
• skip existing AGENTS.md without overwriting;
• keep init without --dry-run blocked until write mode exists;
• preserve check console, JSON and Markdown behavior;
• redact secret-like repository values in init errors;
• add CLI and unit tests for init planning.

Validation:

• ./scripts/check.sh passed with 43 tests;
• check console output verified;
• check JSON output verified;
• check Markdown output verified;
• init --dry-run on a new repository does not write AGENTS.md;
• init --dry-run on an existing AGENTS.md does not overwrite it;
• init without --dry-run returns exit 2;
• init dry-run invalid path returns exit 2;
• secret-like repository values are redacted;
• no raw secret-like literals in changed files.