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

docs/reference/core.md

@@ -50,8 +50,12 @@ specify init my-project --integration copilot --preset compliance
 
 | Variable          | Description                                                              |
 | ----------------- | ------------------------------------------------------------------------ |
+| `SPECIFY_INIT_DIR` | Target a member project from outside its directory (e.g. a monorepo root) without `cd`, for non-interactive / CI use. Set it to the **project root** — the directory *containing* `.specify/` (relative paths resolve against the current directory). The path must exist and contain `.specify/`, otherwise the command errors and does **not** fall back to the current directory. Resolved once in the core root helper (`get_repo_root` in Bash, `Get-RepoRoot` in PowerShell), so it is honored by the core feature scripts (`/speckit.plan`, `/speckit.tasks`, …) and the Git extension's feature-branch creation, which inherit it. When unset, the project is detected by searching upward from the current directory as before. |
+| `SPECIFY_FEATURE_DIRECTORY` | Override the active feature directory *within* the resolved project (takes precedence over `.specify/feature.json`). Relative paths resolve under the project root. Combine with `SPECIFY_INIT_DIR` to pick both the project and the feature non-interactively. |
 | `SPECIFY_FEATURE` | Override feature detection for non-Git repositories. Set to the feature directory name (e.g., `001-photo-albums`) to work on a specific feature when not using Git branches. Must be set in the context of the agent prior to using `/speckit.plan` or follow-up commands. |
 
+> **Two resolution axes.** `SPECIFY_INIT_DIR` selects the **project** (which directory contains `.specify/`); `SPECIFY_FEATURE_DIRECTORY` / `.specify/feature.json` select the **feature** within that project. They are independent — project first, then feature.
+
 ## Check Installed Tools
 
 ```bash