…8802)

## Summary

Three code paths spawn subprocesses that can invoke mise-managed tools
(e.g. `gh`, `git`) without stripping mise shims from PATH. When the tool
resolves to a mise shim, it re-enters mise, which may trigger the same
subprocess again — causing **infinite recursion (fork bomb)**.

Observed: load average >1800 on an ARM SBC, system unresponsive. Also
reproduced on macOS.

## Root Cause

Three subprocess-spawning paths inherit shims in PATH:

### 1. `credential_command` in `src/github.rs`
`get_credential_command_token()` runs `sh -c <cmd>` (e.g. `gh auth
token`). If `gh` is a mise shim → recursion.

### 2. `git credential fill` in `src/github.rs`
`get_git_credential_token()` runs `git credential fill`. If `git` is a
mise shim, or git's credential helper invokes `gh` (via `gh auth
setup-git`) → recursion.

### 3. `exec()` template function in `src/tera.rs` (primary trigger)
When a `.mise.toml` contains:
```toml
[env]
GITHUB_TOKEN = "{{exec(command='gh auth token')}}"
```
Every `mise hook-env` in that directory runs `gh auth token` via
`tera_exec()` with `PRISTINE_ENV`, which includes shims in PATH → `gh`
shim → `mise exec` → evaluates env → runs `gh auth token` again →
infinite recursion.

This is the most common trigger because `exec()` in `[env]` is the
idiomatic way to derive env vars from CLI tools.

## Fix

Add a shared `file::path_env_without_shims()` helper (next to the
existing `which_no_shims()`) that filters `dirs::SHIMS` out of PATH and
returns an `OsString` suitable for `.env("PATH", ...)`. Used in all
three call sites:

- **`src/github.rs`**: `get_credential_command_token()` and
`get_git_credential_token()`
- **`src/tera.rs`**: `tera_exec()`

Follows the same shim-stripping pattern established in:
- PR #8475 (`dependency_env()`)
- PR #8276 / #8560 (`exec_program()`)
- PR #8189 (Windows variant)
- PR #8402 (`uv` venv creation via `which_no_shims()`)

Related: Discussion #6374 — same user-facing symptom ("Cannot fork")
from `exec()` path.

## Reproduction

### Tera exec path (most common)
1. Install `gh` via mise (`gh = "latest"`)
2. Create a project `.mise.toml` with `[env] GITHUB_TOKEN =
"{{exec(command='gh auth token')}}"`
3. `cd` into that directory — `mise hook-env` fires, evaluates the
template, runs `gh auth token` through the shim → fork bomb

### Credential command path
1. Install `gh` via mise, set `credential_command = "gh auth token"` in
mise settings
2. Trigger any GitHub API call → fork bomb

## Changes

- **`src/file.rs`**: New `path_env_without_shims()` public helper —
shared by all call sites
- **`src/github.rs`**: Use shared helper in
`get_credential_command_token()` and `get_git_credential_token()`
- **`src/tera.rs`**: Use shared helper in `tera_exec()`
- **`e2e/cli/test_github_credential_shim_recursion`**: e2e test for
credential_command path
- **`e2e/cli/test_tera_exec_shim_recursion`**: e2e test for tera exec()
path

## Note on env var workaround

Setting `MISE_GITHUB_TOKEN=""` does **not** prevent the hang —
`resolve_token()` filters empty strings with `.filter(|t|
!t.is_empty())`, so it falls through to subprocess-based fallbacks. A
non-empty sentinel value (e.g. `MISE_GITHUB_TOKEN="none"`) works for the
github.rs paths but the tera `exec()` path is independent of token
resolution entirely.

## Test plan

- [x] `cargo check` passes
- [ ] CI: `cargo test` passes  
- [ ] CI: e2e test `test_github_credential_shim_recursion` passes
- [ ] CI: e2e test `test_tera_exec_shim_recursion` passes
- [x] Manual test: project with `exec(command='gh auth token')` in
`.mise.toml`, `cd` does not fork-bomb

---------

Co-authored-by: autofix-ci[bot] <114827586+autofix-ci[bot]@users.noreply.github.com>