fix: handle '%' in filenames when expanding templates

[ethanstoner]

Bug

Formatting or running :ConformInfo on a buffer whose path contains a % (e.g. my%20notes.md) fails with:

runner.lua:521: invalid capture index

Root cause

string.gsub treats % in a string replacement as special (%0-%9 are capture references, %% is a literal percent). The file path was passed straight in as the replacement string in two places:

• build_context: template:gsub("$RANDOM", ...):gsub("$FILENAME", basename) when building the temp file name (stdin = false)
• build_cmd: $FILENAME, $DIRNAME and $EXTENSION replacements when args is a string

$RELATIVE_FILEPATH already used a function replacement, so it was unaffected. List-style args do not use gsub and were also fine.

Change

Use function replacements for those gsub calls. The return value of a replacement function is inserted literally, so % in paths no longer gets interpreted. No other gsub calls in the plugin take path-derived replacement strings.

Tests

Added to tests/runner_spec.lua:

• build_context temp file name with a %20 filename and directory (failed with invalid capture index before the fix)
• build_cmd string args $FILENAME $DIRNAME $EXTENSION with a %20 path (failed with invalid capture index before the fix)
• build_cmd list args with a %20 path (already passed; regression guard)

./run_tests.sh on nvim 0.12 (macOS): all new tests pass. Three tests fail identically on unmodified master in my environment, so they are unrelated to this change: runner e2e sets the correct output, fuzzer formats correctly, injected formatter inline.ts.

stylua --check lua tests and luacheck lua tests pass.

Fixes #725

Generated with Claude Code on behalf of @ethanstoner.