Improve frontmatter diagnostics for invalid tools config shape

[Copilot]

Syntax/configuration diagnostics were low quality for malformed tool config: the compiler pointed at the wrong line, surfaced parser-internal wording, and sometimes attached non-syntax recovery hints. This change makes the error location, message, and hint category align with the actual authoring mistake.

• Diagnostic targeting for scalar-with-children YAML mistakes

  • In frontmatter error formatting, detect the case where a tools.<name> key is set to a scalar but followed by nested keys.
  • Re-anchor the reported position to the offending scalar line/value (e.g. github: "invalid-string") instead of the child key line.

• Human-readable message for known tool-shape violation

  • Replace parser-internal wording for this pattern with a tool-aware message:
    • tools.github tool config must be an object, not a string (for example: toolsets: [default])

• Context-specific recovery classification

  • Expand syntax classification patterns so YAML syntax/tool-shape parser failures receive syntax remediation guidance, avoiding unrelated event/filter hints.

• Focused regression coverage

  • Add/extend tests for:
    • invalid tools.github scalar + nested key caret/message behavior,
    • YAML translation of the parser phrase,
    • syntax error prioritization/hint selection.

# before: caret/message landed on toolsets line with parser-internal text
tools:
  github: "invalid-string"
    toolsets: [default]

# now: reports on github value with actionable guidance
# tools.github tool config must be an object, not a string

[github-actions]

🤖 PR Triage — Run §28715668077

Field	Value
Category	bug
Risk	🟡 Medium
Score	58/100
Impact	28/50
Urgency	18/30
Quality	12/20
Action	batch_review
Batch	pr-batch:go-bugfix

Improves frontmatter diagnostics for invalid tools config shape. +137 lines new error file, +4/-1 Go, tests included. Medium complexity.

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