Problem

psql supports shell command substitution in prompts via ````command```:

PROMPT1='`hostname`:\u@\h:\d> '
PROMPT2='... '

rpg does not execute the backtick-quoted commands — it renders them literally. This was reported by the pspg author (issue #703), who relies on this feature.

Spec

Match psql behaviour:

• In PROMPT1/PROMPT2, detect command (backtick-delimited)
• Execute the command via shell, capture stdout
• Substitute the output (trimmed) into the prompt string
• Execute once per prompt render
• If the command fails or is empty, substitute empty string (no crash)

Also verify other prompt escape sequences work correctly:

• %u — username
• %h — host
• %d — database
• %> — port
• %# — # for superuser, > otherwise
• %R — query buffer state

Implementation hints

• src/repl/mod.rs or wherever prompt is rendered — find the prompt interpolation function
• Add a render_prompt_backtick(cmd: &str) -> String helper
• Use std::process::Command to run the shell command
• Security: run exactly the command the user configured — no additional sandboxing (same as psql)

Acceptance criteria

• PROMPT1='\hostname`:\u@\h:\d> '` renders correctly with actual hostname
• Multi-command prompts work
• Failed commands produce empty substitution, not a crash
• Tests cover: valid command, failing command, empty output, no backtick (regression)
• Existing prompt tests pass