feat(conformance): add FormatMismatchHint to StoryboardStepHint taxonomy

[bokelley]

Closes #947

Summary

• Extends StoryboardStepHint from ContextValueRejectedHint to ContextValueRejectedHint | FormatMismatchHint
• Adds FormatMismatchHint interface with: kind, message, tool, instance_path (RFC 6901 pointer — renamed from issue's field_path to match SchemaValidationError.instance_path convention), expected_format, and optional observed_value
• New pure detection module format-mismatch-hints.ts (parallel to rejection-hints.ts): fires only on strict_only_failure validations — lenient Zod passed, strict AJV rejected on keyword === 'format'
• Runner wires detectFormatMismatchHints as a separate parallel pass outside the stepFailed guard, since format hints are most valuable when the step is green (silent strictness gap)
• Exports FormatMismatchHint and detectFormatMismatchHints from the public storyboard and testing indexes
• 13 unit tests covering: gate logic, pointer decoding (RFC 6901 ~0/~1 escapes, array indices), observed_value extraction and truncation, expected_format regex fallback, edge cases

What's not changing

• Pass/fail is unaffected — hints[] stays non-fatal
• ValidationResult.warning string still fires (unchanged) — the hint is additive
• No protocol-side change; this is a runner diagnostic contract
• Existing detectContextRejectionHints gate (stepFailed) is unchanged

Implementation notes

• instance_path: renamed from field_path (issue spec) to instance_path to align with SchemaValidationError.instance_path — this type is a filtered projection of that type
• expected_format: parsed from AJV's stable message template must match format "<name>" via regex; falls back to '(unknown)' on mismatch (avoids confusing must match format "format" when using the keyword as fallback)
• observed_value: resolved by converting RFC 6901 pointer → dot-bracket notation, then resolvePath(). Strings only, truncated at 200 chars via code-point slicing. Non-string values (objects, arrays, numbers) omitted
• No stepFailed guard needed: skipped/probe/webhook-assertion steps have empty validations[] so no hints emit

Nits noted (not fixed)

• pointerToDotPath in format-mismatch-hints.ts duplicates normalizeFieldPath in rejection-hints.ts. Both could be extracted to path.ts. Deferring to keep this PR scoped.

What was tested

• TypeScript compiles clean (pre-existing @types/node + moduleResolution env errors only — same as PR feat(conformance): add ShapeDriftHint to StoryboardStepHint taxonomy #937)
• prettier --check passes
• 13/13 new unit tests pass (node --test test/lib/storyboard-format-mismatch-hints.test.js)
• Baseline test pass/fail count unchanged vs main (496 failures, all pre-existing dist/build-artifact issues)

Pre-PR review

• ad-tech-protocol-expert: approved — 1 blocker fixed (field_path → instance_path); nit on pointerToDotPath duplication noted above; no protocol change confirmed; minor changeset classification correct
• code-reviewer: approved — no blockers; fixed misleading (unknown) fallback and added guard comment; added 13 unit tests covering all non-trivial cases flagged in review

Session: https://claude.ai/code/session_01DuuYnsXwSs3znAi9b3bbHH

[bokelley]

Superseded by #959 — landed there via strict-AJV partitioning in .

[bokelley]

Superseded by #959 — FormatMismatchHint landed there via strict-AJV partitioning in strict-validation-hints.ts.