Problem

Path-valued flags inside an oasdiff config file are resolved against the current working directory, not the directory containing the config file. This breaks the --config <path> workflow when the config file isn't in cwd.

Reproduction

$ oasdiff breaking \
    data/openapi31-ref-sibling-base.yaml \
    data/openapi31-ref-sibling-revision.yaml \
    --config examples/.oasdiff.yaml

Error: can't process err ignore file: open ignore-err-example.txt: no such file or directory

examples/.oasdiff.yaml contains err-ignore: ignore-err-example.txt. The file exists at examples/ignore-err-example.txt. The CLI looks for it at <cwd>/ignore-err-example.txt — which doesn't exist.

Expected behaviour

Most tools in this category resolve relative paths in config files against the config file's own directory, not against cwd:

• ESLint: paths in .eslintrc are relative to the config file
• Prettier: same
• golangci-lint: same (per docs)

This makes a config file self-contained — wherever it lives, the auxiliary files it references are colocated and resolution Just Works.

Affected flags

Anything in the Config struct (internal/viper.go) that takes a path:

• err-ignore
• warn-ignore
• severity-levels
• template
• (any future path-valued flag added under viper binding)

Why this matters now

#899 (feat(config): rename default config to .oasdiff.*; add --config flag and OASDIFF_CONFIG env var) makes --config <path> and OASDIFF_CONFIG first-class. With those, customers will routinely point at config files that aren't in cwd — exactly the case where cwd-relative resolution silently fails. The lookup-mechanism PR doesn't fix this; it surfaces it.

Proposed fix

When a config-file path is resolved (whether from --config, OASDIFF_CONFIG, or the default cwd lookup), determine its directory. Path-valued flags read from that file are resolved against that directory unless they're already absolute.

Roughly, after ReadInConfig in internal/viper.go::readConfFile:

1. Capture the resolved config file path: viper.GetViper().ConfigFileUsed() returns the absolute path of the file that was loaded.
2. After validate() / before flag binding, walk the path-valued fields in Config (or just specific keys: err-ignore, warn-ignore, severity-levels, template); if the value is a relative path, rewrite it to filepath.Join(filepath.Dir(configPath), value).
3. Don't rewrite when the config file is in cwd (the rewrite is a no-op in that case anyway, but worth confirming behaviour doesn't subtly change for current users).

Backward compatibility

This is a semantic change for any existing user who relied on cwd-relative resolution while using a config file in a non-cwd location. Realistically:

• Default-cwd-lookup users (.oasdiff.yaml next to where they run): no change. Config dir == cwd, paths resolve identically.
• --config <path> users: behaviour shifts. Today they need absolute paths or to invoke from the same directory; after this change, paths in their config work as colocated.

The shift is from "broken-by-default for --config" to "works-by-default for --config" — an improvement, but technically a semantic change. Worth calling out in release notes.

Implementation scope

• internal/viper.go — ~20-line change
• New test cases in internal/viper_test.go covering: relative path in config file resolves correctly when config is in cwd vs --config vs OASDIFF_CONFIG; absolute paths in config are not rewritten.
• Documentation update in docs/CONFIG-FILES.md and examples/.oasdiff.yaml.

Out of scope

• Resolving $variables inside config-file paths (env-var expansion). Separate ergonomic concern; defer.
• Upward search for config files in parent directories. Separate concern; defer.

Related

• #899 — the lookup-mechanism PR that surfaces this issue.