feat(api): Probe for @ mentions in TUI append to prompt API

[ewired]

1.0 removed the ability for POST /tui/append-prompt to include @ mentioned files and agents. This patch extracts and uses the mention resolution for files and agents used by command templates, marking the mentions as if they were added from Autocomplete. The behavior of the append prompt TUI command is also changed to move the cursor to the end before inserting, and I don't know if pre-1.0 versions did this. Unlike Autocomplete, it can resolve files relative to the home directory mentioned with ~/, which might be undesired.

Fixes #2129

[ewired]

The only failing test is failing on other recent PRs and pertains to the patch tool which is apparently dead code.

[ewired]

The original approach of this PR is incompatible with opencode after 4bb7ea9 because an Instance instance is apparently no longer provided to the TUI renderer or TUI command handlers. This can be based on the new SessionPrompt.resolvePromptParts method added from #4221 but it also uses Instance.

[ewired]

The new approach adds the ability to specify parts on TuiEvent.PromptAppend and must extract the mentions using the Instance information during the response to /tui/append-prompt. This means /tui/append-prompt has different capabilities than TuiEvent.PromptAppend and couldn't be a direct pass-through anymore.

[github-actions]

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