feat(shellcheck): support configurable path exclusions for shell script discovery

[josecelano]

Problem

find_shell_scripts() in src/linters/shellcheck.rs currently hard-codes only two exclusions:

"-not", "-path", "*/.git/*",
"-not", "-path", "*/.terraform/*",

Any directory that contains vendored or generated shell scripts (e.g. .tmp/, target/, a local Cargo registry cache) will be scanned even if it is listed in .gitignore. This breaks linter all in projects that temporarily materialise external shell scripts inside the workspace.

Concrete example

In torrust/torrust-tracker, a cold-run benchmark script uses an isolated CARGO_HOME placed under .tmp/workflow-benchmarks/ to approximate a shared-runner first run. After the run, .tmp/ contains vendored .sh files from crates such as zerocopy and sqlx. Those files contain shellcheck errors that are not ours to fix, so linter shellcheck (and therefore linter all) exits non-zero.

.tmp/ is already listed in .gitignore, but the linter does not consult .gitignore.

Proposed solution

Add support for a project-level exclusion file — .linterignore or a [shellcheck] section in a linter.toml — that lists additional path patterns to pass as -not -path arguments to the internal find call.

A minimal approach that would unblock the immediate use case:

1. If a .linterignore file exists in the working directory, read it (one glob pattern per line, # comments stripped).
2. For each pattern, append -not -path "<pattern>" to the find invocations in both find_shell_scripts() for .sh and .bash files.
3. Apply the same exclusion list to other linters that also call find internally (e.g. markdownlint, cspell) if they have the same problem.

A longer-term alternative is to respect .gitignore directly (e.g. via git ls-files --others --exclude-standard to discover untracked scripts, or by using the ignore crate).

Acceptance criteria

• Projects can exclude one or more directory patterns from shell script discovery without modifying the linter source.
• The exclusion mechanism is documented in the README.
• linter all passes in torrust/torrust-tracker with .tmp/ in the exclusion list.
• Existing behaviour is unchanged when no exclusion config is present.

Context

• Affects: src/linters/shellcheck.rs → find_shell_scripts()
• Related repo: torrust/torrust-tracker (issue #1841 — workflow performance baseline analysis)
• Discovered while running cold-run benchmark scripts that write an isolated CARGO_HOME under .tmp/