fix(#904): normalize phase number in init.execute-phase branch_name (#909)

Wrap the {phase} substitution in normalizePhaseName() at both fix sites:
- src/init.cts  — cmdInitExecutePhase branch_name output
- src/commands.cts — cmdCommit pre-execution branch derivation

When project_code is set (e.g. "CK"), extractPhaseToken returns the
full prefixed token "CK-01" as phase_number.  Without normalization the
generated branch was "gsd/phase-CK-01-foundation"; after this fix it is
"gsd/phase-01-foundation", matching the documented {phase} contract
(padded numeric only).

Adds a regression test in tests/init.test.cjs.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

src/commands.cts

@@ -547,7 +547,7 @@ function cmdCommit(cwd: string, message: string | undefined, files: string[] | u
         const phaseInfo = findPhaseInternal(cwd, phaseNum) as Record<string, unknown> | null;
         if (phaseInfo) {
           branchName = (config['phase_branch_template'] as string)
-            .replace('{phase}', phaseInfo['phase_number'] as string)
+            .replace('{phase}', normalizePhaseName(phaseInfo['phase_number']))
             .replace('{slug}', (phaseInfo['phase_slug'] as string) || 'phase');
         }
       }

src/init.cts

@@ -260,7 +260,7 @@ function cmdInitExecutePhase(
       config.branching_strategy === 'phase' && phaseInfo
         ? (config.phase_branch_template as string)
             .replace('{project}', (config.project_code as string) || '')
-            .replace('{phase}', phaseInfo['phase_number'] as string)
+            .replace('{phase}', normalizePhaseName(phaseInfo['phase_number']))
             .replace('{slug}', (phaseInfo['phase_slug'] as string) || 'phase')
         : config.branching_strategy === 'milestone'
           ? (config.milestone_branch_template as string)