fix(agent-context): discover nested plan.md in scoped layouts (#3024) (#3301)

* fix(agent-context): discover nested plan.md in scoped layouts (#3024)

The agent-context updater only looked for plan.md one level deep
(specs/*/plan.md), so scoped layouts created via SPECIFY_FEATURE_DIRECTORY
(specs/<scope>/<feature>/plan.md) were never picked up and no plan
reference was written into the context file.

Recurse into specs/ in both the bash (rglob) and PowerShell (-Recurse)
scripts. In the PowerShell script, also replace
[System.IO.Path]::GetRelativePath, which is .NET Core 2.1+ only and throws
under Windows PowerShell 5.1 (.NET Framework); the exception was swallowed
by the surrounding try/catch, leaving the plan path empty on 5.1 even when
a plan was found. Compute the project-relative path by stripping the root
prefix instead.

Add regression tests for both scripts covering nested discovery.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

* Potential fix for pull request finding

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

* fix(agent-context): guard mtime plan discovery against symlink escape

Address Copilot review feedback on #3301:

- bash updater: the mtime fallback filtered candidates lexically via
  relative_to() on the *unresolved* path, so a plan reached through a
  specs/ symlink pointing outside the project could be selected and emit
  an in-project-looking path. Resolve each candidate and keep only those
  whose resolved path stays under root before picking the newest.
- test: the nested-plan PowerShell regression targets a Windows
  PowerShell 5.1 (.NET Framework) failure mode, but ran whatever
  POWERSHELL resolved to (prefers pwsh). Prefer powershell.exe on Windows
  so the 5.1-only compat fix is actually exercised.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

* docs(agent-context): note recursive plan.md discovery in update command

Auto-detection now recurses (`specs/**/plan.md`) to support nested scoped layouts created via SPECIFY_FEATURE_DIRECTORY (#3024). The update command doc still described the old one-level `specs/*/plan.md` glob, which could mislead users troubleshooting plan detection. Addresses Copilot review feedback on PR #3301.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

* Potential fix for pull request finding

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

---------

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

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

@@ -426,7 +426,11 @@ if (-not $PlanPath) {
     if (-not $PlanPath) {
         try {
             $specsDir = Join-Path $ProjectRoot 'specs'
-            $candidate = Get-ChildItem -Path $specsDir -Recurse -File -Filter 'plan.md' -ErrorAction SilentlyContinue |
+            # Recurse (rather than the old one-level specs/*/plan.md scan) so scoped
+            # layouts created via SPECIFY_FEATURE_DIRECTORY, e.g.
+            # specs/<scope>/<feature>/plan.md, are still discovered when
+            # feature.json is absent (#3024).
+            $candidate = Get-ChildItem -Path $specsDir -Filter 'plan.md' -File -Recurse -ErrorAction SilentlyContinue |
                 Sort-Object LastWriteTime -Descending |
                 Select-Object -First 1
             if ($candidate) {