fix: find plans in nested spec directories (#3405)

* fix: find plans in nested spec directories

The agent-context mtime fallback used a one-level specs/*/plan.md
glob, so scoped layouts (specs/<scope>/<feature>/plan.md via
SPECIFY_FEATURE_DIRECTORY) never matched and the SPECKIT block was
written without a plan path. Recurse in both script variants and
update the command doc wording.

Fixes #3024

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* refactor: pick newest plan with max(), align doc wording

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

---------

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

extensions/agent-context/scripts/powershell/update-agent-context.ps1

@@ -12,7 +12,7 @@
 #
 # When `plan_path` is omitted, the script derives it from `.specify/feature.json`
 # (written by /speckit-specify). Falls back to the most recently modified
-# `specs/*/plan.md` only when feature.json is absent or its plan does not exist yet.
+# `specs/**/plan.md` only when feature.json is absent or its plan does not exist yet.
 
 [CmdletBinding()]
 param(
@@ -426,9 +426,7 @@ if (-not $PlanPath) {
     if (-not $PlanPath) {
         try {
             $specsDir = Join-Path $ProjectRoot 'specs'
-            $candidate = Get-ChildItem -Path $specsDir -Directory -ErrorAction SilentlyContinue |
-                ForEach-Object { Get-Item -LiteralPath (Join-Path $_.FullName 'plan.md') -ErrorAction SilentlyContinue } |
-                Where-Object { $_ } |
+            $candidate = Get-ChildItem -Path $specsDir -Recurse -File -Filter 'plan.md' -ErrorAction SilentlyContinue |
                 Sort-Object LastWriteTime -Descending |
                 Select-Object -First 1
             if ($candidate) {