Add dispatch info <id> for a concise session summary

[jongio]

Adds dispatch info <id>, a concise one-screen summary of a single session. export dumps every turn; info shows just the shape.

What changed

• info <id> prints ID, summary, repository, branch, directory, host, created/updated/last-active timestamps, and counts for turns, files, checkpoints, and references grouped into commits, PRs, and issues.
• Empty optional fields are omitted; counts always print.
• info <id> --json emits the same data as a machine-readable object.
• A missing ID errors clearly; an unknown ID reports not found.
• The detail loader is shared with export, so no new store code is added.
• Wired into handleArgs, the usage banner, and all four shell completion lists (bash, zsh, fish, powershell).

Testing

• go build ./..., go vet ./..., go test ./... -count=1 all green.
• golangci-lint run reports 0 issues; mage deadcode OK.
• New unit tests cover arg parsing, the reference count rollup, text output, JSON output, empty-field omission, and error paths.

Also drains the stdout pipe on a goroutine in TestPrintUsage_Output so the grown usage banner cannot deadlock on the smaller Windows pipe buffer.

Closes #312