feat: open slash command menu automatically with settings opt-out

[Brian Krabach (bkrabach)]

What changed

• Open the top-level slash-command menu automatically when the user types a leading /.

• Keep the behavior on by default; absent or malformed settings retain the enabled default.

• Add the ui.slash_popup.enabled opt-out in ~/.amplifier/settings.yaml:

  ui: {slash_popup: {enabled: false}}

• With enabled: false, only the automatic popup is disabled. Tab completion, command arguments, aliases, and history search remain available.

• Resolve the setting once when each interactive session is constructed, including resumed sessions; no discovery or I/O is added to keypress handling.

Why

Slash commands are easier to discover without requiring a first Tab, while users who prefer the prior quieter interaction can restore Tab-only behavior with one explicit setting.

Verification

• Full suite: 2,158 passed, 1 skipped, 13 deselected, 1 expected failure.
• Integration suite: 13 passed.
• Targeted slash-completion tests: 37 passed.
• Configured Ruff, git diff --check, and production-source hash checks passed.
• Final live terminal gate passed for all three settings states: missing setting (default enabled), ui.slash_popup.enabled: false, and ui.slash_popup.enabled: true.
• The isolated fixture-environment setup corrected the earlier guard/setup failure; the source was unchanged. Disabled and enabled sessions passed automatic-popup, Tab-only completion, navigation, argument cycling, and Enter/Esc behavior. No prose, provider command, model prompt, or memory call was submitted.

Breaking changes

None. Automatic popup is additive and can be disabled without changing command or argument completion behavior.