Placeholder System Overhaul (v1.4.0)

### Background

The placeholder/formatting engine is the most heavily used subsystem of `generate-release-notes`, yet it has accumulated fragmented requests across multiple releases. Several individual placeholder features have been requested (#76, #78, #80, #189, #205) alongside a known default-value defect (#297) and a broader architectural proposal (#226) to unify the system.

This epic consolidates all placeholder-related work into a single coherent release, ensuring users get a well-documented, consistent placeholder API after v1.3.0.

### Goals

- Fix the known defect where the progress placeholder is absent from the default value configuration (#297)
- Deliver a robust, unified placeholder system and output formatting engine (#226)
- Add all identified missing placeholders (release notes text, contributor template, new placeholders) (#80, #76, #205)
- Give users full control over row-format placeholder prefixes and add prefix/postfix support (#78, #189)
- Ensure all new/changed placeholders are covered by tests and documented

### Subtasks

- [ ] #297 Progress placeholder is not present in default value
- [ ] #226 Feature: Robust Placeholder System and Output Formatting Engine
- [ ] #205 Add new placeholder
- [ ] #189 Add prefix and postfix to placeholders
- [ ] #80 Placeholder(s) for release notes text
- [ ] #78 Row format placeholder prefix - add user control
- [ ] #76 Add a placeholder for format template for all contributors

### Related / References

- Follows v1.3.0 release
- #226 serves as the architectural umbrella for this epic