Reshape proposal-review skill around its Slack output

[ewels]

Summary

The nfcore-proposal-review skill was structured as a general "upstream" skill plus a runtime addendum that overrode the body for the Slack-bot context — which is the only context the skill ever runs in. This reshapes it around that single reality.

• Folded the addendum into the body. Tooling (run_command, bundled scripts, gh/jq/nf-core), the "proposal is already provided" note, and the Slack message format are now the primary spec rather than overrides tacked on the end.
• One canonical output structure — the Slack verdict message + a single draft GitHub comment code block. Dropped the old verdict-block + analysis-narrative format that the addendum overrode anyway, and removed the batch-sweep mode (Slack always reviews one proposal).
• De-shouted the prose — verdict tokens and status markers to title case (e.g. CONDITIONAL_ACCEPT → Conditional Accept, PASS/FAIL → Pass/Fail), underscores to spaces in prose. Code identifiers, URLs, and acronyms (CI, SIG, DRAGEN, GxP) left as-is.
• Fixed invalid YAML frontmatter. The description's Triggers on: "..." colon-space parsed as a nested mapping key. Switched to a folded block scalar (>-), matching the slack-app skill convention.

Test plan

• Frontmatter parses as valid YAML (verified with a YAML parser)
• Spot-check a live proposal review renders correctly in Slack

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