feat(query): `--follow` to subscribe to a storage value and print on change

Part of #226 (live chain observation). Ship this first — smallest viable streaming slice.

What

Add a `--follow` flag to storage queries so that instead of a one-shot read, the CLI subscribes to the storage item and prints the value each time it changes.

dot polkadot.query.System.Number --follow
dot asset-hub.query.System.Account <addr> --follow

Why

Proves the subscription plumbing with minimal new surface — reuses the existing storage key construction + decode path in `src/commands/query.ts`. No new dashboard/TUI needed. Unblocks the most common "why can't I watch for changes?" need.

Approach

• Use papi's storage `watchValue` / observable subscription instead of the one-shot read, behind the `--follow` flag (default off = current behavior, no regression).
• Reuse the existing decode/format path so followed output matches one-shot output.
• Lifecycle: run until SIGINT (Ctrl-C); tear down the subscription and client cleanly on exit.

Output contract

• TTY: print each new value (timestamped / labeled) as it arrives.
• `--json`: newline-delimited JSON, one object per change, drain-safe via `writeStdout`. Depends on #162 (valid JSON for empty values).

Out of scope

• Multiplexing many keys in one invocation (compose multiple processes for now).
• The full block/event dashboard — that's `dot watch` (separate child of #226).

Verification

• `dot .query.System.Number --follow` prints a new number roughly every block.
• Without `--follow`, behavior is byte-for-byte the existing one-shot output (regression check).
• `--follow --json | jq` parses cleanly line-by-line.
• Ctrl-C exits 0 and closes the WS connection (no dangling socket).

Start in src/commands/query.ts and trace the existing storage key construction, decode, and format path. Add the --follow flow around papi's watchValue or observable subscription while preserving the one-shot path, then verify regular and --json output, repeated changes, and clean Ctrl-C shutdown with the commands listed in the issue.