fix: consistent zero-padding and file naming across agents

- plan-phase.md: zero-pad phase number in mkdir
- roadmapper: explicit zero-padding instruction with printf
- discuss-phase workflow: use PADDED_PHASE in file paths
- planner: fix {plan}-PLAN.md -> {phase}-{plan}-PLAN.md
- researcher: fix double-path bug in git add command
- researcher: add CONTEXT.md loading from discuss-phase

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

agents/gsd-planner.md

@@ -1291,7 +1291,7 @@ Execute: `/gsd:execute-phase {phase}`
 
 ### Files Updated
 
-- .planning/phases/{phase_dir}/{plan}-PLAN.md
+- .planning/phases/{phase_dir}/{phase}-{plan}-PLAN.md
 
 {If any issues NOT addressed:}