feat(repl): /dream manual command + report rendering

[smallnest]

Summary

• Intercept /dream and /dream --dry-run in the REPL loop (mirrors /compact /memory): spawn the process-isolated pigo --dream [--dream-dry-run] -C <cwd> subprocess, show a progress line, capture the single-line Report JSON on stdout, and render a full-table change report.
• Full table: merged / deduped / paths-cleaned / pruned / distilled, bytes + files before→after, reconciled counts, and Notes. --dry-run reflects report.DryRun with a clear DRY-RUN label ("nothing written").
• Error handling per SPEC §6.1: a pre-spawn read-only lock check surfaces "已有 dream 在运行" when a background dream holds a live lock; subprocess exit 1 or unparseable stdout prints a clear error without crashing the REPL. Subprocess bounded by a 10min timeout (SPEC §6.3).
• RenderReportLine(report) exported for reuse by the startup background trigger (dream: startup background due-trigger + one-line summary #526). Added /dream to the /help command list.

Closes #525

Test plan

• go build ./... && go vet ./... clean
• go test ./internal/cli/repl/... ./internal/dream/... ./internal/cli/prompts/... green
• Renderer unit tests: full table + one-line, DRY-RUN label, zero-value, byte formatting
• Parse+render path via spawn seam with canned JSON (no real LLM), failure path, lock-held notice