fix: accept 4+ digit spec numbers in tests and docs (#2094)

Two test assertions in test_timestamp_branches.py used the regex
`\d{3}` (exactly 3 digits) instead of `\d{3,}` (3 or more digits).
While the underlying shell scripts already handle spec numbers ≥ 1000
correctly — printf "%03d" and PowerShell '{0:000}' both expand naturally
beyond 3 digits, and all detection regexes use {3,} — the overly-strict
test assertions would fail with a misleading error if a fixture ever
contained 1000+ spec directories.

Documentation in README.md, spec-driven.md, and the CLI --branch-numbering
help text implied that sequential spec numbers are always 3 digits, which
could lead users to believe a hard limit of 999 exists.

Changes:
- tests/test_timestamp_branches.py: change two \d{3} assertions to \d{3,}
- src/specify_cli/__init__.py: clarify help text to show numbers expand past 999
- README.md: update --branch-numbering docs to note numbers expand beyond 3 digits
- spec-driven.md: update feature numbering description to include 4-digit example

Fixes #2093

Co-authored-by: alex-zwingli <alex-zwingli@users.noreply.github.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

spec-driven.md

@@ -78,7 +78,7 @@ The SDD methodology is significantly enhanced through three powerful commands th
 
 This command transforms a simple feature description (the user-prompt) into a complete, structured specification with automatic repository management:
 
-1. **Automatic Feature Numbering**: Scans existing specs to determine the next feature number (e.g., 001, 002, 003)
+1. **Automatic Feature Numbering**: Scans existing specs to determine the next feature number (e.g., 001, 002, 003, …, 1000 — expands beyond 3 digits automatically)
 2. **Branch Creation**: Generates a semantic branch name from your description and creates it automatically
 3. **Template-Based Generation**: Copies and customizes the feature specification template with your requirements
 4. **Directory Structure**: Creates the proper `specs/[branch-name]/` structure for all related documents