fix: support feature branch numbers with 4+ digits (#2040)

* fix: support feature branch numbers with 4+ digits in common.sh and common.ps1

The sequential feature number pattern was hardcoded to exactly 3 digits
(`{3}`), causing branches like `1234-feature-name` to be rejected.
Changed to `{3,}` (3 or more digits) to support growing projects.
Also added a guard to exclude malformed timestamp patterns from being
accepted as sequential prefixes.

Closes #344

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* fix: narrow timestamp guard and use [long] to prevent overflow

- Change [int] to [long] in PowerShell Get-CurrentBranch to avoid
  overflow for large feature numbers (>2,147,483,647)
- Narrow malformed-timestamp exclusion from ^[0-9]+-[0-9]{6}- to
  ^[0-9]{7}-[0-9]{6}- so valid sequential branches like
  004-123456-fix-bug are not rejected

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* test: add regression tests for 4+ digit feature branch support

Cover check_feature_branch and find_feature_dir_by_prefix with 4-digit
sequential prefixes, as requested in PR review #2040.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* fix: reject timestamp-like branches without trailing slug

Branches like "20260319-143022" (no "-<name>" suffix) were incorrectly
accepted as sequential prefixes. Add explicit rejection for 7-or-8
digit date + 6-digit time patterns with no trailing slug, in both
common.sh and common.ps1.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

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Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>