fix(cli): render StoryboardStepResult.hints[] in all reporting surfaces

[bokelley]

Summary

Wires StoryboardStepResult.hints[] into every human-readable and CI reporting surface that was silently dropping it. The runner already populates hints with context_value_rejected diagnostics (landed in #875/#870); this PR makes them visible.

Also extracts formatStoryboardResultsAsJUnit from bin/adcp.js into src/lib/testing/storyboard/junit.ts (marked @internal) so the formatter can be unit-tested without bootstrapping the full CLI dependency tree.

What changed

bin/adcp.js

• adcp storyboard run (single + multi-instance): prints Hint: <message> below the Error: line for each failing step with hints
• adcp storyboard step: same, matching that command's no-indent error style
• Adds comment at each hint block documenting the runner invariant (hints only emitted on failed steps)
• Removes the inline formatStoryboardResultsAsJUnit function (now imported from dist/lib)

src/lib/testing/storyboard/junit.ts (new, @internal)

• Hints included in <failure> body between step.error and failing validations
• message= attribute falls back to step.hints?.[0]?.message when step.error is absent — so CI systems that only read the attribute still surface the diagnostic
• Named export makes it reachable from tests; @internal JSDoc and absence from top-level barrel signal it is not part of the public @adcp/client API

Test plan

• node --test test/lib/storyboard-junit-hints.test.js — 5/5 pass (new snapshot tests)
• node --test test/lib/storyboard-rejection-hints.test.js — 21/21 pass (existing)
• node --test test/lib/storyboard-runner-contract.test.js — 29/30 pass (1 pre-existing skip unchanged)
• npm run format:check — clean
• CI green

Note for reviewer: comply path

Planning experts flagged whether to also thread hints through ComplianceFailure so they appear in adcp comply human-readable output. The dx-expert called this a follow-up concern; the protocol-expert called it out-of-scope (the comply JSON path already serializes hints verbatim). This PR takes the narrower scope per the issue's stated acceptance criteria.

Nits noted (not fixed)

• No test for XML-special characters in hint messages (the xmlEscape call is correct; a test would pin it)
• message= attribute omits the Hint: prefix intentionally for brevity (pinned by test on line 108 of storyboard-junit-hints.test.js)

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Closes #879

Pre-PR review:

• code-reviewer: approved — no blockers; documented runner invariant in CLI hint blocks per review
• dx-expert: approved after fix — resolved one blocker (@internal added to exported symbol to clarify public/internal boundary); 2 nits noted above

Session: https://claude.ai/code/session_01CokHw4pHKBK2v1aYVE9LfL

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[bokelley]

Closing as obsolete — the core hint-rendering work in this PR landed via #891 before this branch was rebased. The three remaining deltas (extract formatStoryboardResultsAsJUnit into a library module, wire hints into adcp storyboard step, and message= attribute fallback to hints?.[0]?.message) are being cherry-picked into a fresh, smaller PR branched off current main to avoid a messy conflict resolution.