chore: prepare v0.3.1 release

[ningzimu]

Summary

• move current unreleased changelog entries into the 0.3.1 release section
• classify the slide subagent fix under Fixes

Verification

• python3 -m py_compile skills/codex-ppt/scripts/prepare_slide_prompts.py skills/codex-ppt/scripts/record_slide_blocker.py skills/codex-ppt/scripts/record_slide_dispatch.py skills/codex-ppt/scripts/record_slide_result.py skills/codex-ppt/scripts/slide_job_status.py skills/codex-ppt/scripts/slide_run_state.py
• git diff --check

[ningzimu]

Quick pass done.

• this PR is release bookkeeping only (CHANGELOG.md)
• moving current unreleased entries into 0.3.1 and classifying the slide subagent item under Fixes looks right
• existing script compile command from the PR body is still the right smoke check for the freshly merged slide-subagent helpers

No blocker from me.

[ningzimu]

Heads-up before cutting v0.3.1: Codex review on merged PR #31 surfaced two plausible follow-up bugs in the newly added slide-state helpers:

1. record_slide_dispatch.py marks a slide as dispatched without checking current active dispatch count against max_concurrent_slides, so state can oversubscribe beyond the configured cap.
2. record_slide_result.py accepts --backend-used if it matches any expected label source, which can let inconsistent backend labels silently pass and weaken the fixed-backend-after-sample-approval contract.

I spot-checked both paths on main; they look worth addressing before shipping the release payload from #31 as 0.3.1.