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>

src/specify_cli/__init__.py

@@ -840,7 +840,7 @@ def init(
     ai_skills: bool = typer.Option(False, "--ai-skills", help="Install Prompt.MD templates as agent skills (requires --ai)"),
     offline: bool = typer.Option(False, "--offline", help="Deprecated (no-op). All scaffolding now uses bundled assets.", hidden=True),
     preset: str = typer.Option(None, "--preset", help="Install a preset during initialization (by preset ID)"),
-    branch_numbering: str = typer.Option(None, "--branch-numbering", help="Branch numbering strategy: 'sequential' (001, 002, ...) or 'timestamp' (YYYYMMDD-HHMMSS)"),
+    branch_numbering: str = typer.Option(None, "--branch-numbering", help="Branch numbering strategy: 'sequential' (001, 002, …, 1000, … — expands past 999 automatically) or 'timestamp' (YYYYMMDD-HHMMSS)"),
     integration: str = typer.Option(None, "--integration", help="Use the new integration system (e.g. --integration copilot). Mutually exclusive with --ai."),
     integration_options: str = typer.Option(None, "--integration-options", help='Options for the integration (e.g. --integration-options="--commands-dir .myagent/cmds")'),
 ):