feat: [ENG-2851] add curate response envelope to restore HITL for tool-mode curate

Both MCP brv-curate and CLI session protocol now carry a structured
`{html, meta?}` envelope. The calling agent's LLM supplies operation
metadata (type, impact, reason, summary) alongside the HTML. Both
surfaces converge through a new shared `buildCurateHtmlLogEntry`
helper that persists a CurateLogEntry via FileCurateLogStore — so
`brv review pending`, `brv status`, push aggregation, and the TUI
review surface all light up for tool-mode curates again.

Out of scope: dream HITL (already works), previousSummary
auto-derivation, DELETE operations, additionalFilePaths, review-UI
changes, curate-completion notification broadcast.