feat(scripts): add SPECIFY_INIT_DIR to target a member project from the repo root (#2892)

* feat(scripts): add SPECIFY_INIT_DIR to target a member project from the repo root

Resolve an explicit SPECIFY_INIT_DIR project override once in the core
get_repo_root / Get-RepoRoot, so a non-interactive / CI caller can target a
member project (the directory containing .specify/) from a monorepo root
without cd. Strict by design: the path must exist and contain .specify/,
otherwise it hard-errors with no silent fallback.

- Single resolver in core; the git feature-branch script inherits it by
  sourcing core, with no per-extension copies.
- PS resolver verifies the resolved path is a directory (Resolve-Path also
  succeeds for files) so a file value errors as "not an existing directory".
- get_feature_paths splits decl/assignment so a SPECIFY_INIT_DIR failure
  propagates instead of being masked by `local`.
- create-new-feature-branch: when core is absent (only git-common loaded) and
  SPECIFY_INIT_DIR is set, hard-error rather than silently using the git root.
- Document SPECIFY_INIT_DIR and SPECIFY_FEATURE_DIRECTORY in the core reference.
- Tests for valid/relative/trailing-slash/file/missing/no-.specify targets,
  feature-axis composition, the no-core guard, and a PowerShell mirror.

* fix: guard SPECIFY_INIT_DIR with stale core scripts

* docs: clarify SPECIFY_FEATURE_DIRECTORY precedence wording

* fix: normalize trailing slash in PowerShell SPECIFY_INIT_DIR resolver

Resolve-Path preserves a trailing separator from its input, so a
SPECIFY_INIT_DIR ending in a slash returned a root that didn't match the
bash resolver (whose `cd && pwd` strips it). That broke
test_ps_trailing_slash_tolerated on the CI runners, which do have pwsh.
Trim it with TrimEndingDirectorySeparator (no-op on a bare root or a path
with no trailing separator).

Also fix the misleading test comment: the PowerShell mirror runs on the
CI ubuntu/windows runners (they ship pwsh), it is not skipped there.

* test: normalize bash path expectations on Windows

* docs: clarify SPECIFY_INIT_DIR root helpers

extensions/git/scripts/powershell/create-new-feature-branch.ps1

@@ -197,7 +197,16 @@ if (-not $commonLoaded) {
     throw "Unable to locate common script file. Please ensure the Specify core scripts are installed."
 }
 
-# Resolve repository root
+# SPECIFY_INIT_DIR is resolved (and validated) by the core resolver. If only the
+# minimal git-common.ps1 was loaded, or an older core common.ps1 without the
+# resolver was loaded, refuse rather than silently falling back to the wrong root.
+if ($env:SPECIFY_INIT_DIR -and -not (Get-Command Resolve-SpecifyInitDir -CommandType Function -ErrorAction SilentlyContinue)) {
+    throw "SPECIFY_INIT_DIR requires updated Spec Kit core scripts (common.ps1 with Resolve-SpecifyInitDir), which were not found."
+}
+
+# Resolve repository root. When the core scripts are present, Get-RepoRoot
+# honors SPECIFY_INIT_DIR (the explicit project override for non-interactive /
+# CI use) and hard-fails on an invalid value with no silent fallback.
 if (Get-Command Get-RepoRoot -ErrorAction SilentlyContinue) {
     $repoRoot = Get-RepoRoot
 } elseif ($projectRoot) {