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>

---------

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

scripts/bash/common.sh

@@ -78,7 +78,7 @@ get_current_branch() {
                         latest_timestamp="$ts"
                         latest_feature=$dirname
                     fi
-                elif [[ "$dirname" =~ ^([0-9]{3})- ]]; then
+                elif [[ "$dirname" =~ ^([0-9]{3,})- ]]; then
                     local number=${BASH_REMATCH[1]}
                     number=$((10#$number))
                     if [[ "$number" -gt "$highest" ]]; then
@@ -124,9 +124,15 @@ check_feature_branch() {
         return 0
     fi
 
-    if [[ ! "$branch" =~ ^[0-9]{3}- ]] && [[ ! "$branch" =~ ^[0-9]{8}-[0-9]{6}- ]]; then
+    # Accept sequential prefix (3+ digits) but exclude malformed timestamps
+    # Malformed: 7-or-8 digit date + 6-digit time with no trailing slug (e.g. "2026031-143022" or "20260319-143022")
+    local is_sequential=false
+    if [[ "$branch" =~ ^[0-9]{3,}- ]] && [[ ! "$branch" =~ ^[0-9]{7}-[0-9]{6}- ]] && [[ ! "$branch" =~ ^[0-9]{7,8}-[0-9]{6}$ ]]; then
+        is_sequential=true
+    fi
+    if [[ "$is_sequential" != "true" ]] && [[ ! "$branch" =~ ^[0-9]{8}-[0-9]{6}- ]]; then
         echo "ERROR: Not on a feature branch. Current branch: $branch" >&2
-        echo "Feature branches should be named like: 001-feature-name or 20260319-143022-feature-name" >&2
+        echo "Feature branches should be named like: 001-feature-name, 1234-feature-name, or 20260319-143022-feature-name" >&2
         return 1
     fi
 
@@ -146,7 +152,7 @@ find_feature_dir_by_prefix() {
     local prefix=""
     if [[ "$branch_name" =~ ^([0-9]{8}-[0-9]{6})- ]]; then
         prefix="${BASH_REMATCH[1]}"
-    elif [[ "$branch_name" =~ ^([0-9]{3})- ]]; then
+    elif [[ "$branch_name" =~ ^([0-9]{3,})- ]]; then
         prefix="${BASH_REMATCH[1]}"
     else
         # If branch doesn't have a recognized prefix, fall back to exact match