feat(spec): implement concrete release resolution for workspace specs

Workspace directories are created on demand by `crud.create_workspace()` (which calls `manager.workspaces_dir / <name>.mkdir()`). The `ensure_paths()` method creates the top-level `workspaces/` directory at startup, but individual workspace subdirectories are created only when a workspace is created.

### Spec Release Resolution Must Be Concrete

Spec releases must always be resolved to a **full 3-part version** (e.g. `"18.0.0"`) before being stored or used for file resolution. Ambiguous selectors like `"latest"`, `"18"`, or `"18.0"` are resolved via database lookup at the **earliest possible point** — at `workspace add`-time via `resolve_spec_release()` in `specs/operations/checkout.py`.

**Why:** The spec checkout directory (`checkout/Specs/archive/{series}/{spec}/`) is **shared across all releases** of a spec. After downloading REL19, files for REL18 and REL19 coexist as version-specific subdirectories (e.g. `26261-g00/` for REL19, `26261-i00/` for REL18). If a downstream function receives an unresolved `"latest"` and scans the whole tree, it can return files from the wrong release.

**Two-layer contract:**

1. **Resolve early** — `resolve_spec_release()` pins the concrete version at add-time, embedding it in the workspace member ID (e.g. `26261-REL18.0.0`).

2. **Respect downstream** — `fetch_spec_files()` uses the pinned version to scan only the version-specific extracted directory, never the shared parent.

**Rule:** Never pass ambiguous release selectors to fetch/convert functions. Never scan a shared checkout directory without filtering by release.

### Function Renaming Follows Scope

When a function's scope changes, rename it to match. Example: `delete_ai_folder()` became `delete_artifact_folder()` when it stopped being `.ai`-specific and started cleaning generic workspace artifact directories. Names should describe current behavior, not historical behavior.

# Summary - 2026-05-02 - SPEC_RELEASE_RESOLUTION

## Problem

When adding a spec to a workspace multiple times with different releases (e.g. `--release latest` then `--release 18.0`), both workspace source directories ended up containing **identical files** from the wrong release. For example, `26261-REL18.0.0/` and `26261-REL19.0.0/` both contained REL19 files.

## Root Cause

Two bugs in the spec fetch pipeline:

1. **`fetch_spec_files()` ignored the requested release when scanning local files.** The spec checkout directory (`checkout/Specs/archive/{series}/{spec}/`) is shared across all releases. After REL19 was downloaded, requesting REL18 triggered a local scan of the entire tree, which returned REL19 files without checking the release.

2. **`_download_spec()` discarded the version-specific extracted directory path.** `checkout_specs_async()` returns `list[Path]` of version-specific subdirectories (e.g. `26261-i00/` for REL18), but the return value was ignored. The subsequent full-tree scan could return files from any release.

## Changes

### 1. Early Release Resolution at Add-Time

Added `resolve_spec_release()` in `specs/operations/checkout.py` — resolves ambiguous release selectors (`"latest"`, `"18"`, `"18.0"`) to full 3-part versions (e.g. `"18.0.0"`) via database lookup before the workspace member is created.

- Added `AutoCrawlSpecsOption` (`--auto-crawl-specs/--no-auto-crawl-specs`) to `cli/args.py`.

- Updated `workspace_add` in `cli/_workspace_commands.py` to resolve spec releases at add-time.

- Workspace member IDs now always contain a concrete release: `26261-REL18.0.0`.

Impacted files: `specs/operations/checkout.py`, `cli/args.py`, `cli/_workspace_commands.py`.

### 2. Version-Specific File Fetching

Fixed `fetch_spec_files()` in `extraction/fetch_spec.py` to respect concrete release versions:

- `_download_spec()` now returns the version-specific extracted directory `Path` (e.g. `.../26261-i00/`) instead of discarding it.

- `fetch_spec_files()` skips the local tree scan for concrete releases, downloads the correct version, and scans only the version-specific extracted directory.

- Local-first scan is preserved for generic selectors (`"latest"`, `"all"`, `None`).

Impacted file: `extraction/fetch_spec.py`.

## Design Principle

See `docs/development.md` — "Spec Release Resolution Must Be Concrete":

- **Resolve early**: Pin concrete version at add-time via `resolve_spec_release()`.

- **Respect downstream**: Fetch/convert functions must scan only version-specific directories, never the shared checkout tree.

## Verification

- Ruff lint: all checks passed.

- Pyright: no new diagnostics.

- 79/79 tests passed.