feat(cli): mid-message slash-command completion #1100

Problem

slashCommandPrefix (packages/cli/src/pi-tui-pickers.ts) only triggers completion when the text before the cursor starts with / at column 0 of the line, so typing / mid-message (after whitespace) yields no command completion.

Desired outcome

/ triggers command completion wherever it begins a token (line start or after whitespace), without hijacking absolute paths like /Users/.... Completion only — execution semantics stay unchanged: a message runs as a command only when it starts with /.

Status

• Done elsewhere: listing / selecting skills for explicit invocation shipped as /skill picker + /skill:<name> tokens in feat: explicit skill invocation in TUI and desktop, aligned on a shared runtime contract #1148 / feat: explicit skill invocation via /skill:<name> tokens in the TUI #1150 (not the /skills dialog originally sketched here). That half of the original dual-gap issue is closed by those PRs.
• Still open (this issue): mid-message slash-command completion for the general command surface only.

(Split out of the retired #549 tracker; skill half superseded by #1148.)