docs: add v1.1.0 frontmatter enhancements and modular spec structure

- Add date_format frontmatter field for custom output formatting
- Add field behavior table (locale, timezone, date_format)
- Document file-scoped frontmatter (no inheritance to linked files)
- Split specification.md into modular detail files:
  - spec/frontmatter.md: field behavior, processing, file scope
  - spec/examples.md: date/time, YAML, multi-file examples
  - spec/libraries.md: library stack and processing pipeline
  - spec/compatibility.md: todo.txt, todo.md, xit conversion
- Reduce specification.md from 656 to 508 lines (under AISD 600 limit)
- Configure semantic-release for docs-first repos (docs=minor)
- Auto-update version strings in markdown files on release
- Populate CHANGELOG.md with v1.0.0 release notes

Author: ricksaarge

CHANGELOG.md

@@ -1 +1,26 @@
-# 1.0.0 (2025-12-05)
+# Changelog
+
+## [1.0.0](https://github.com/taskmark/taskmark-spec/releases/tag/v1.0.0) (2025-12-05)
+
+Initial release of the TaskMark specification.
+
+### Features
+
+- Markdown-based task format (.md files)
+- Four task states: `[ ]` open, `[x]` done, `[-]` cancelled, `[!]` blocked
+- Flexible priorities `(A-Z)` and numeric `(1-99)`
+- Assignees with `@user` syntax
+- Hierarchical projects `+project/sub` and tags `#tag`
+- ISO 8601 datetime support with timezones
+- Recurring tasks with natural language patterns (`repeat:weekly`)
+- Section-based metadata inheritance
+- Multi-file organization with markdown links
+- Subtasks (one level deep)
+- YAML frontmatter for file-level settings (`locale`, `timezone`)
+
+### Documentation
+
+- Complete technical specification
+- Format compatibility guide (todo.txt, todo.md, xit)
+- AI agent development guide (AISD-inspired)
+- Library stack recommendations