…w error wrapping (#1929)

## Summary

Addresses the High and Medium priority findings from the
[BurntSushi/toml Go module
review](#1849).

## Changes

### Remove dead `*toml.ParseError` pointer assertion (`config_core.go`)

`toml.Decode` always returns `ParseError` as a **value type**, so the
`err.(*toml.ParseError)` pointer assertion could never match. This was
misleading dead code with a stale comment claiming it was needed for
"compatibility".

### Use `%w` wrapping for richer parse error messages

Replaced:
```go
return nil, fmt.Errorf("failed to parse TOML at line %d, column %d: %s",
    perr.Position.Line, perr.Position.Col, perr.Message)
```

With:
```go
return nil, fmt.Errorf("failed to parse TOML: %w", perr)
```

This surfaces `ParseError.Error()`'s full formatted output — including
the TOML source line and a `^` column pointer — giving users much better
context when their config has a syntax error. It also preserves the
structured `ParseError` type for callers using `errors.As`.

**Before:**
```
failed to parse TOML at line 5, column 12: expected value but found 'x'
```

**After:**
```
failed to parse TOML:

    5 | port = x
               ^
Error: expected value but found 'x'
```

### Update package doc comment

Reflects the new error-wrapping approach instead of the old manual field
extraction.

## Testing

All unit and integration tests pass (`make agent-finished`).