config: use ParseError.ErrorWithUsage() for richer TOML startup diagnostics

[Copilot]

When config.toml fails to parse, the startup log only showed the terse one-liner from err.Error(). toml.ParseError.ErrorWithUsage() (available since BurntSushi/toml v1.6.0) produces a source-context-rich diagnostic with a file snippet and column pointer — significantly better for on-call debugging.

Changes

• internal/config/config_core.go — adds exported FormatConfigError(err error) string that unwraps toml.ParseError via errors.As and returns ErrorWithUsage(), falling back to err.Error() for all other error types. Keeps the BurntSushi/toml import confined to the config package.

• internal/cmd/root.go — replaces err.Error() with config.FormatConfigError(err) in the LogErrorToMarkdown call on config load failure.

• internal/config/config_core_test.go — three subtests: TOML parse error produces |/^ source annotations, plain error falls back correctly, wrapped non-TOML error falls back correctly.

Before:

Configuration validation failed:
failed to parse TOML: toml: line 5 (field command): expected "=", got "[" instead

After:

Configuration validation failed:
toml: line 5 (field command): expected "=", got "[" instead

  3 | [servers.github]
  4 | command = "docker"
  5 | [servers.github
      | ^

[Copilot]

Pull request overview

This PR improves startup diagnostics when config.toml fails to parse by emitting BurntSushi/toml’s source-context-rich ParseError.ErrorWithUsage() output instead of the plain err.Error(), making on-call debugging significantly easier.

Changes:

• Added config.FormatConfigError(err error) string to unwrap TOML toml.ParseError and return ErrorWithUsage() when applicable.
• Updated startup error logging in internal/cmd/root.go to use config.FormatConfigError(err).
• Added unit tests covering TOML parse errors vs non-TOML error fallbacks.

Show a summary per file

File	Description
internal/config/config_core.go	Adds FormatConfigError helper to format TOML parse errors with source context.
internal/cmd/root.go	Uses FormatConfigError when logging config load failures during startup.
internal/config/config_core_test.go	Adds tests verifying FormatConfigError behavior across TOML and non-TOML errors.

Review details

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• Files reviewed: 3/3 changed files
• Comments generated: 1
• Review effort level: Low