Slash command autocomplete should trigger mid-line, not only at the start of the prompt #40719

Problem

In the TUI prompt, / only opens the command autocomplete menu when it is the first character of the input. Typing:

fix the bug then /commit

does not surface command suggestions at /commit. You have to move the cursor to the start, or know the command name with no completion help.

This is inconsistent with the existing @ mention trigger, which already works anywhere in the line when preceded by whitespace.

Proposed approach

Mirror mentionTriggerIndex for /:

• Extract a shared tokenTriggerIndex(value, offset, trigger) helper from mentionTriggerIndex (DRY, no parallel logic).
• Add slashTriggerIndex reusing the same whitespace-preceded rule, so foo /comp triggers but https://x, foo/bar, MM/DD do not. Newline counts as a separator; Unicode display width handled via the existing promptOffsetWidth path.
• Make server-command onSelect and hide() index-aware so selecting a command mid-line replaces only the typed token, not the whole line.
• Reopen the menu when the cursor steps back onto a / token anywhere in the line, mirroring the @ reopen path.

The start-of-line case (store.index === 0) is preserved unchanged; the mid-line path is purely additive.

Edge case worth flagging

/ appears in prose more often than @ (paths, URLs, dates, division). The whitespace-preceded rule rejects https:// and foo/bar, but reviewers may want to weigh in on extra guards.

Verification

• I have verified this behavior does not exist in the latest version
• I have searched for existing issues and PRs that cover this

Working implementation exists locally, ~40 lines across packages/tui/src/prompt/display.ts and packages/tui/src/component/prompt/autocomplete.tsx, plus a slashTriggerIndex test suite mirroring the existing mention-trigger cases. Happy to open the PR once there is sign-off on the direction.