Closes #276.

Summary

Real-world JSONC files (knip.jsonc, tsconfig.json, .vscode/settings.json, etc.) routinely use trailing commas. load_json_or_jsonc ran the input through json_comments::StripComments and then handed the result to serde_json, which rejects trailing commas with trailing comma at line N column M — exactly the symptom in the issue.

Fix

Add a final pass that drops , immediately before } or ] while leaving commas inside string literals untouched. The pass runs only when the comment-stripped parse fails, so already-valid input pays zero cost on the happy path:

1. Try plain serde_json::from_str — works for normal JSON.
2. Strip comments via json_comments::StripComments, retry parse.
3. If that still fails, run strip_trailing_commas on the comment-stripped text and retry.

strip_trailing_commas is a small byte-level state machine (no new deps) that walks the input, tracks in-string + escaped state, and emits the original text minus any , that's followed only by whitespace before } or ]. Multi-byte UTF-8 chars are preserved verbatim because their continuation bytes never collide with the , / \" / \\ / } / ] markers we look at.

Tests

crates/cli/src/migrate/tests.rs adds:

• load_json_or_jsonc_accepts_trailing_commas — reproduces the issue exactly: a knip.jsonc shaped file with trailing commas after "src/main.ts" and "dist/**" and the outermost object closer. Was failing before, parses correctly now.
• load_json_or_jsonc_handles_comments_and_trailing_commas_together — // line comment, /* block comment */, and trailing commas all in the same file.
• Six unit tests on the helper itself: simple object/array trailing comma, separator-comma preservation, comma-in-string preservation ("hello, world,"), escaped quotes ("he said \"hi,\","), and pretty-printed-with-newlines input.

cargo test -p fallow-cli — 1745 passed, 0 failed.

Backwards compatibility

Strict JSON inputs hit branch 1 and bypass the new pass entirely, so anything that worked before still works. The new pass only runs when both branch 1 and branch 2 already failed, so it's a strictly additive fallback.