refactor(prompt): simplify Buzz agent guidance (#6340)

## Why
The Buzz agent prompt carries verbose agent-draft instructions that add
prompt weight without improving benchmark correctness. Buzz-native
channel trials also showed agents creating permanent channels while
claiming they had applied a one-hour TTL. The static CLI inventory
remains for efficient command discovery by smaller agents.

## What
- Retain the static Buzz CLI command table and direct agents to use
`--help` for full usage
- Compact the owner-reviewed agent-draft guidance without changing its
workflow
- Clarify that `buzz channels create` is permanent unless `--ttl` is
provided

## Benchmark Results
- Terminal-Bench retained 22/22 correctness in the initial no-table
candidate while reducing prompt size, input tokens, tool calls, and
cost; total active time was effectively flat.
- Across two Buzz-native runs, the non-channel cases tied at 26/30; the
CLI clarification directly addresses the observed TTL omission.

## Update — 2026-08-20
- Restored the CLI command table following review feedback about
multistep discovery with smaller agents. The restored-table candidate
has not been re-benchmarked.

Signed-off-by: Salman Mohammed <smohammed@squareup.com>

crates/buzz-cli/src/lib.rs

@@ -582,8 +582,9 @@ pub enum ChannelsCmd {
         /// Channel description
         #[arg(long)]
         description: Option<String>,
-        /// Make the channel ephemeral: lifetime in seconds. The relay archives
-        /// it once this many seconds pass without a new message.
+        /// Make the channel temporary/ephemeral: idle lifetime in seconds. If
+        /// omitted, the channel is permanent. The relay archives it once this
+        /// many seconds pass without a new message.
         #[arg(long, value_name = "SECONDS")]
         ttl: Option<i64>,
         /// Apply a desktop-local channel template by name (case-insensitive):