feat(cli): add channels search for human-readable channel lookup

[tlongwell-block]

Summary

Adds sprout channels search --query <name> so humans and agents can resolve a channel UUID from a human-readable name without opening channel settings or guessing.

Acceptance from the brief in cli-channel-lookup:

• ✅ Search by partial and exact channel name.
• ✅ Output includes UUID and display name.
• ✅ Works for visible channels, including private channels the caller can see (relay-side access control on kind:39000 already enforces this).
• ✅ JSON output consistent with existing CLI style.

What's in the diff

• New ChannelsCmd::Search { query, exact, include_archived, limit } variant in crates/sprout-cli/src/lib.rs.
• New cmd_search_channels + ChannelSummary::from_event + name_matches in crates/sprout-cli/src/commands/channels.rs, plus a dispatch arm.
• 7 unit tests covering tag parsing (happy path, archived, private, missing required tags, malformed tags) and the name-match predicate.

Stable JSON projection per result:

{
  "channel_id": "…",
  "name": "…",
  "channel_type": "stream" | "forum" | …,
  "visibility": "public" | "private",
  "archived": false,
  "about": "…" | null,
  "topic": "…" | null,
  "purpose": "…" | null
}

Sorted by name, then channel_id for determinism.

Design notes

• Source of truth is kind:39000 (NIP-29 group metadata). The relay emits it from side_effects::emit_group_discovery_events with name, about, t, private/public, archived, topic, purpose. The existing channels list/get query kind:39002 (membership) and so have no name tag at all — addressing that is intentionally out of scope here to avoid breaking scripts.
• NIP-50 search is not used. The Typesense index sets query_by: "content", and kind:39000 events have empty content — the name lives in tags. A plain {kinds:[39000]} query plus client-side post-filter is simpler and correct.
• No new auth. crates/sprout-relay/src/handlers/req.rs already runs per-event accessible_channels checks on globally-filtered queries, so private channels the caller can't see are dropped server-side.
• Limit default 1000. The DB clamps un-tuned query_events at 1000 anyway; making it explicit avoids surprise truncation.
• --exact returns all case-insensitive exact matches. Channel names are not unique (no DB UNIQUE constraint, no "name taken" path on the relay — only repos have that), so the flag narrows, not necessarily to one.

Example

$ sprout channels search --query cli
[
  {"channel_id":"d71e3abe-…","name":"cli-channel-lookup","channel_type":"stream","visibility":"private",…},
  {"channel_id":"7bca45ee-…","name":"cli-refactor-and-migration","channel_type":"stream","visibility":"public",…}
]

Verification

• cargo build -p sprout-cli ✓
• cargo test -p sprout-cli --lib channels — 7/7 ✓
• cargo clippy -p sprout-cli --all-targets -- -D warnings ✓
• cargo fmt -p sprout-cli --check ✓
• Live tested against the relay: substring, --exact, --query "" (rejected), unknown-query → [], --help.

Review

Plan was iterated to 9/10+ with Max (GPT-5.5) before implementation; diff was independently reviewed by Max (build, clippy, fmt, tests, live behavior, DCO trailer) and signed off. Thread: https://github.com/block/sprout channel cli-channel-lookup.

Follow-ups (not in this PR)

• channels list / channels get currently return raw kind:39002 events with no name. Worth switching to kind:39000 in a small follow-up, but it changes the JSON shape and is intentionally separated to avoid breaking scripts in this PR.