fix: accept 4+ digit spec numbers in tests and docs

[alex-zwingli]

Summary

Fixes #2093

The shell scripts already handle spec numbers ≥ 1000 correctly — printf "%03d" and PowerShell '{0:000}' expand naturally beyond 3 digits, and all detection regexes throughout common.sh, common.ps1, and create-new-feature.* use {3,} quantifiers. However, two issues remained:

1. Overly-strict test assertions

Two assertions in tests/test_timestamp_branches.py used \d{3} (exactly 3 digits):

# Before
assert re.match(r"^\d{3}-new-feat$", branch)
assert re.match(r"^\d{3}-seq-feat$", branch)

# After
assert re.match(r"^\d{3,}-new-feat$", branch)
assert re.match(r"^\d{3,}-seq-feat$", branch)

These tests pass today because their fixtures start from low spec numbers, but the hard-coded \d{3} would produce a misleading failure if a test context ever had 1000+ specs — rather than confirming the scripts work correctly with 4-digit prefixes.

2. Documentation implied a 3-digit hard limit

Three docs described sequential numbering exclusively with 3-digit examples (001, 002, 003), implying a hard cap of 999 specs. Updated to clarify that numbers expand naturally past 3 digits.

Changes

File	Change
tests/test_timestamp_branches.py	\d{3} → \d{3,} in two assertions
src/specify_cli/__init__.py	CLI --branch-numbering help text updated
README.md	--branch-numbering table row updated
spec-driven.md	Feature numbering description updated

Testing

All existing tests pass:

uv run pytest tests/test_timestamp_branches.py tests/test_branch_numbering.py
49 passed, 5 skipped

The existing test_sequential_supports_four_digit_prefixes test (introduced previously) confirms the scripts work end-to-end with a 4-digit transition (999 → 1000 → 1001).

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AI disclosure: This PR was written primarily by GitHub Copilot.

[Copilot]

Pull request overview

Updates tests and documentation to correctly reflect that sequential spec/branch numbering supports 4+ digit prefixes (e.g., 1000+) rather than implying a hard 3-digit limit, aligning assertions and user-facing text with existing script behavior.

Changes:

• Relax sequential branch-name test assertions from \d{3} to \d{3,} to allow 1000+ prefixes.
• Clarify --branch-numbering help/documentation to state sequential numbering expands past 999 automatically.
• Update SDD documentation to include examples beyond 3 digits.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
tests/test_timestamp_branches.py	Updates two regex assertions to accept 3+ digit sequential prefixes.
src/specify_cli/__init__.py	Adjusts --branch-numbering help text to clarify 1000+ behavior.
README.md	Updates CLI options table description for --branch-numbering to include 1000+ examples.
spec-driven.md	Updates feature numbering description to explicitly mention expansion beyond 3 digits.

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