cli/sdk: `burn search <query>` over FTS5 content

[willwashburn]

Parent: #240
Split out from: #248 (see scope-update comment carried over from #259)

Context

burn search <query> was added to the CLI scope in the #259-driven scope update on #248 but never landed. Neither the SDK verb nor the CLI presenter exists today:

• No search / search_* symbol in crates/relayburn-sdk/src/lib.rs.
• No Search variant in crates/relayburn-cli/src/cli.rs::Command.

Scope

SDK (relayburn-sdk)

• New verb (e.g. relayburn_sdk::search) backed by an FTS5 virtual table over the content.sqlite body store.
• Inputs: query string, optional session_id, optional limit, optional snippet flag.
• Output: ranked hits with session id, turn id, and (when snippet) a highlighted excerpt.

CLI (relayburn-cli)

• New burn search <query> [--session <id>] [--limit N] [--snippet] subcommand.
• Thin presenter over the SDK verb. Honors --json for machine output and the global --ledger-path override.

Acceptance

• FTS5 virtual table is created/migrated as part of the content.sqlite schema (and rebuilt by burn state rebuild --content-only).
• relayburn_sdk::search returns ranked hits against the cli-golden fixture ledger.
• burn search <query> golden-output test passes (human + --json modes).
• --snippet returns highlighted excerpts; --limit caps result count; --session <id> scopes to one session.