feat(web): complete the leading slash command with Tab in the composer

[whitelonng]

What

Tab in the composer now completes the highlighted leading / command as plain text instead of the browser's focus walk: typing /go + Tab fills /goal into the draft, keeps focus, and runs nothing until Enter. Picking was deliberately avoided — the menu pick path executes bare (argument-less) commands immediately.

Behavior

• Leading / + highlighted candidate + Tab → splice /<name> over the token span (scoped slash/input-insert-text), close the menu, keep focus.
• Any other open-menu state (candidates still loading, inline token, @ trigger) → Tab is consumed so focus cannot escape the composer.
• Closed menu / IME composition → Tab keeps native focus behavior.
• Enter and pointer picks are unchanged.

Changes

• ArbitrateKey gains tab; InputTriggerController.arbitrate routes tab to a new complete() arm.
• InputBar.onKeyDown intercepts Tab and preventDefaults whenever arbitration is not pass.
• Controller and composer specs pin the completion/consumption matrix; READMEs updated; Agent Note records the decision.

Verification

• test:gui green (330 files / 4557 tests)
• client typecheck, oxlint, doc-sync (28 gates) green