Fail fast in validate-content-rules for missing files and malformed month/year headings

[VatsalSy]

Why

Content validation has been strengthened, but review feedback shows remaining failure modes should be explicit and deterministic.

Evidence

• PR Harden content and script maintenance safeguards #62 (CodeRabbit review): validator should fail early if expected files are missing.
• PR Harden content and script maintenance safeguards #62 (CodeRabbit review): months before any year heading and unknown month names should be hard errors, not silently skipped.

Scope

• Add a reusable file-existence guard for all files referenced by scripts/validate-content-rules.sh.
• In history parsing, treat unknown month names as validation failures.
• Reject month headings that appear before any year context.
• Improve error messages to include offending heading/context.

Acceptance Criteria

• Missing required files produce clear "file not found" messages and non-zero exit.
• Invalid month names and out-of-context month headings fail validation.
• Existing valid history.md ordering passes unchanged.
• Script output is specific enough to fix failures without manual debugging.