Make trace command logging truncation configurable via PREK_LOG_TRUNCATE_LIMIT (#1679)

Trace-level `Executing ...` lines currently truncate command arguments
at a hardcoded 120 characters, which hides critical details when
debugging hooks (notably `docker_image` entrypoint/args). This change
introduces a configurable truncate limit while preserving existing
default behavior.

- **Config surface: new env var**
  - Added `PREK_LOG_TRUNCATE_LIMIT` to `prek-consts` (`EnvVars`).

- **Runtime behavior in command logging**
- Updated `Cmd` display formatting in `crates/prek/src/process.rs` to
read truncation limit from `PREK_LOG_TRUNCATE_LIMIT`.
- Cached the resolved truncate limit with `LazyLock` so env parsing
happens once.
  - Kept default at `120` when unset/invalid.
- Treated `0` as invalid and fallback to default to avoid degenerate
output.

- **Documentation**
- Added `PREK_LOG_TRUNCATE_LIMIT` to `docs/configuration.md` under
environment variables, including default and intent.

- **Focused unit coverage**
  - Added tests for:
    - env var value parsing (valid/invalid/zero),
    - env-based resolution behavior,
    - cached-limit behavior via `LazyLock`,
    - env restoration guard to avoid cross-test env leakage.

```rust
const DEFAULT_COMMAND_LOG_TRUNCATE_LIMIT: usize = 120;
static COMMAND_LOG_TRUNCATE_LIMIT: LazyLock<usize> =
    LazyLock::new(command_log_truncate_limit_from_env);

fn command_log_truncate_limit() -> usize {
    *COMMAND_LOG_TRUNCATE_LIMIT
}
```

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<details>

<summary>Original prompt</summary>

> 
> ----
> 
> *This section details on the original issue you should resolve*
> 
> <issue_title>Do not truncate trace run executions</issue_title>
> <issue_description>### Summary
> 
> I was trying to debug a docker_image hook execution and was delighted
to note the --log-file option.
> 
> However, turns out that the "Executing" TRACE run lines are truncated
with `[...]` before showing the part I'm actually interested in (a case
of tweaking entrypoints and arguments).
> 
> I'd suggest not truncating these lines in the trace log at all, or
providing an option not to truncate.
> 
> ### Willing to submit a PR?
> 
> - [ ] Yes — I’m willing to open a PR to fix this.
> 
> ### Platform
> 
> Ubuntu 25.10 amd64
> 
> ### Version
> 
> 0.3.2
> 
> ### .pre-commit-config.yaml
> 
> N/A
> 
> ### Log file
> 
> ```
> 2026-02-20T10:17:47.223740Z TRACE run{hook_id=...
language=docker_image}: Executing `cd ... && docker run --rm --tty
--user 1000:1000 --init --volume ...:/src:rw,Z --workdir /src
--entrypoint ... --network [...]`
> ```</issue_description>
> 
> <agent_instructions>Add a new env var to prek-consts, controlling the
command line log truncate limit in process.rs, current it is hardcoded
to 120. Document it in docs/configuration.md env vars
section.</agent_instructions>
> 
> ## Comments on the Issue (you are @copilot in this section)
> 
> <comments>
> </comments>
> 


</details>



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- Fixes #1678

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Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: j178 <10510431+j178@users.noreply.github.com>

Author: Copilot

crates/prek-consts/src/env_vars.rs

@@ -28,6 +28,7 @@ impl EnvVars {
     pub const SSL_CERT_FILE: &'static str = "SSL_CERT_FILE";
     pub const PREK_CONTAINER_RUNTIME: &'static str = "PREK_CONTAINER_RUNTIME";
     pub const PREK_QUIET: &'static str = "PREK_QUIET";
+    pub const PREK_LOG_TRUNCATE_LIMIT: &'static str = "PREK_LOG_TRUNCATE_LIMIT";
 
     // PREK internal environment variables
     pub const PREK_INTERNAL__TEST_DIR: &'static str = "PREK_INTERNAL__TEST_DIR";

crates/prek/src/process.rs

@@ -30,13 +30,23 @@ use std::fmt::Display;
 use std::path::Path;
 use std::process::Output;
 use std::process::{CommandArgs, CommandEnvs, ExitStatus, Stdio};
+use std::sync::LazyLock;
 
 use owo_colors::OwoColorize;
+use prek_consts::env_vars::EnvVars;
 use thiserror::Error;
 use tracing::trace;
 
 use crate::git::GIT;
 
+static LOG_TRUNCATE_LIMIT: LazyLock<usize> = LazyLock::new(|| {
+    EnvVars::var(EnvVars::PREK_LOG_TRUNCATE_LIMIT)
+        .ok()
+        .and_then(|limit| limit.parse::<usize>().ok())
+        .filter(|limit| *limit > 0)
+        .unwrap_or(120)
+});
+
 /// An error from executing a Command
 #[derive(Debug, Error)]
 pub enum Error {
@@ -495,7 +505,7 @@ impl Display for Cmd {
             }
             write!(f, " {}", arg.to_string_lossy())?;
             len += arg.len() + 1;
-            if len > 120 {
+            if len > *LOG_TRUNCATE_LIMIT {
                 write!(f, " [...]",)?;
                 break;
             }

docs/configuration.md

@@ -1355,6 +1355,8 @@ prek supports the following environment variables:
     - `podman`
     - `container` (Apple's Container runtime on macOS, see [container](https://github.com/apple/container))
 
+- `PREK_LOG_TRUNCATE_LIMIT` — Control the truncation limit for command lines shown in trace logs (`Executing ...`). Defaults to `120` characters of arguments; set a larger value to reduce truncation.
+
 Compatibility fallbacks:
 
 - `PRE_COMMIT_ALLOW_NO_CONFIG` — Fallback for `PREK_ALLOW_NO_CONFIG`.