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

scripts/bash/common.sh

@@ -24,9 +24,42 @@ find_specify_root() {
     return 1
 }
 
+# Resolve an explicit SPECIFY_INIT_DIR project override (the directory that
+# *contains* .specify/), for non-interactive / CI use — e.g. running a Spec Kit
+# command against a member project from a monorepo root without cd.
+#
+# Precondition: SPECIFY_INIT_DIR is non-empty. Echoes the validated absolute
+# project root, or prints an error and returns 1. Strict by design: the path
+# must exist and contain .specify/, with no silent fallback to cwd or the
+# script-location default (which would silently write to the wrong project).
+#
+# This is the single resolver: bundled extensions inherit it by sourcing core
+# (e.g. the git extension's create-new-feature-branch) rather than duplicating it.
+resolve_specify_init_dir() {
+    local init_root
+    # Normalize: relative paths resolve against $(pwd); a trailing slash collapses.
+    # CDPATH="" so a relative value cannot be resolved against the caller's CDPATH
+    # (which would also echo to stdout and corrupt the captured path).
+    if ! init_root="$(CDPATH="" cd -- "$SPECIFY_INIT_DIR" 2>/dev/null && pwd)"; then
+        echo "ERROR: SPECIFY_INIT_DIR does not point to an existing directory: $SPECIFY_INIT_DIR" >&2
+        return 1
+    fi
+    if [[ ! -d "$init_root/.specify" ]]; then
+        echo "ERROR: SPECIFY_INIT_DIR is not a Spec Kit project (no .specify/ directory): $init_root" >&2
+        return 1
+    fi
+    printf '%s\n' "$init_root"
+}
+
 # Get repository root, prioritizing .specify directory
 # This prevents using a parent repository when spec-kit is initialized in a subdirectory
 get_repo_root() {
+    # Explicit project override wins (see resolve_specify_init_dir).
+    if [[ -n "${SPECIFY_INIT_DIR:-}" ]]; then
+        resolve_specify_init_dir
+        return
+    fi
+
     # First, look for .specify directory (spec-kit's own marker)
     local specify_root
     if specify_root=$(find_specify_root); then
@@ -119,8 +152,12 @@ _persist_feature_json() {
 }
 
 get_feature_paths() {
-    local repo_root=$(get_repo_root)
-    local current_branch=$(get_current_branch)
+    # Split decl/assignment so a SPECIFY_INIT_DIR validation failure in
+    # get_repo_root propagates as a hard error instead of being masked by `local`.
+    local repo_root
+    repo_root=$(get_repo_root) || return 1
+    local current_branch
+    current_branch=$(get_current_branch)
 
     # Resolve feature directory.  Priority:
     #   1. SPECIFY_FEATURE_DIRECTORY env var (explicit override)