Feature request: run over staged files only, without stashing

[alexjball]

Summary

Add a way to scope a hook run to staged files only without
performing a git stash. Today the only way to get the
"staged files only" file list is to enable a stash method, which
mutates the working tree.

Motivation

In src/hook.rs::file_list(), the file-set decision is binary:

• stash truthy → git_status.staged_files only
  (src/hook.rs:1222-1224)
• stash falsy → staged + unstaged + untracked
  (src/hook.rs:1226-1234)

So "staged files only" is only reachable by also opting into
stash_unstaged / pop_stash, which:

• modifies the working tree mid-run,
• requires capturing/restoring the index,
• and can interact badly with editors, watchers, and other
  concurrent tooling that observes the worktree.

Users who already trust their unstaged changes — or who run hk
outside a pre-commit gate (CI on a checked-out ref, scripted
fixers, IDE integrations) — want the scope without the
mutation.

Reproduction

hk -v run pre-commit --fix --stash none -P

Today this runs over staged + unstaged + untracked files
(no stash, full file set). There is no invocation that combines
"no stash" with "staged files only" — the only way to get the
staged-only file list is to drop --stash none and accept the
worktree mutation.

Observed file-set difference

With --stash none (no stash, full scope — what you get today
when you want to avoid worktree mutation):

$ hk -v run pre-commit --fix --stash none -P
DEBUG $ git update-index -q --refresh
✔ Fetching modified files
DEBUG files: {"project-a/.envrc", "project-a/.gitignore",
"project-a/CLAUDE.md", "project-a/proj.toml",
"project-b/.envrc", "project-b/.gitignore",
"project-b/CLAUDE.md", "project-b/proj.toml",
"project-c/.gitignore", "project-c/CLAUDE.md",
"project-c/proj.toml", "hk.pkl",
"project-d/CLAUDE.md", "project-d/README.md",
"project-e/.gitignore", "project-e/proj.toml",
"project-f/README.md",
"project-g/.envrc", "project-g/.gitignore",
"project-g/CLAUDE.md", "project-g/proj.toml"}
Plan: pre-commit
Run type: fix

  [parallel group] group_0
    ✓ pj-git-check            (21 files matched)
    ✓ check-added-large-files (21 files matched)
    ✓ prettier                (7 files matched)
    ○ ruff                    (no files matched filters)
    ✓ tombi                   (5 files matched)
    ✓ tombi-format            (5 files matched)
    ○ biome                   (no files matched filters)
    ✓ pkl                     (1 file matched)
    ✓ pkl-format              (1 file matched)

With --stash git (staged only, but mutates the worktree —
the only way to get staged-only scope today):

$ hk -v run pre-commit --fix --stash git -P
DEBUG $ git update-index -q --refresh
✔ Fetching staged files
DEBUG files: {"hk.pkl"}
Plan: pre-commit
Run type: fix

  [parallel group] group_0
    ✓ pj-git-check            (1 file matched)
    ✓ check-added-large-files (1 file matched)
    ○ prettier                (no files matched filters)
    ○ ruff                    (no files matched filters)
    ○ tombi                   (no files matched filters)
    ○ tombi-format            (no files matched filters)
    ○ biome                   (no files matched filters)
    ✓ pkl                     (1 file matched)
    ✓ pkl-format              (1 file matched)

Only hk.pkl was actually staged; the other 20 files are
unstaged or untracked changes that --stash none pulls in.
The desired invocation would match --stash git's file set
(just hk.pkl) without touching the worktree.

Proposed behavior

Decouple the "narrow to staged" branch from the stash boolean.
Options, in rough order of preference:

1. New CLI flag: --staged (and matching HookOptions field)
   that selects the staged-only file list, independent of
   --stash.
2. New StashMethod variant: StashMethod::None already
   means "no stash, full file set." A StashMethod::ScopeOnly
   (or similar name) could mean "no stash, staged-only scope."
3. Config-level option on Hook (e.g. scope = "staged")
   mirroring the CLI flag for hooks where this should be the
   default.

Whichever surface is chosen, the implementation is a small
change in file_list(): branch on the new flag before the
existing stash check.

Out of scope

• Changing the default behavior of any existing hook.
• Changing what stash does when enabled.

Workarounds today

• hk check --files "$(git diff --cached --name-only)" — works
  but loses hk's normal file discovery (untracked-but-staged
  intent files, deletions, renames) and is awkward in
  hk.pkl-driven hook configs.
• Per-step glob / condition filters — narrow what each step
  sees but don't change the hook-level file list.