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>

tests/test_timestamp_branches.py

@@ -134,7 +134,7 @@ class TestSequentialBranch:
             if line.startswith("BRANCH_NAME:"):
                 branch = line.split(":", 1)[1].strip()
         assert branch is not None
-        assert re.match(r"^\d{3}-new-feat$", branch), f"unexpected branch: {branch}"
+        assert re.match(r"^\d{3,}-new-feat$", branch), f"unexpected branch: {branch}"
 
     def test_sequential_ignores_timestamp_dirs(self, git_repo: Path):
         """Sequential numbering skips timestamp dirs when computing next number."""
@@ -289,7 +289,7 @@ class TestE2EFlow:
             capture_output=True,
             text=True,
         ).stdout.strip()
-        assert re.match(r"^\d{3}-seq-feat$", branch), f"branch: {branch}"
+        assert re.match(r"^\d{3,}-seq-feat$", branch), f"branch: {branch}"
         assert (git_repo / "specs" / branch).is_dir()
         val = source_and_call(f'check_feature_branch "{branch}" "true"')
         assert val.returncode == 0