Bug: getDirectory() shows a fake "/" parent for root-level files

Description

Three related bugs in packages/core/src/util/path.ts, all from assuming a delimiter is present.

1. getDirectory() invents a parent for root-level files. It joins all-but-last segments and unconditionally appends "/", so a path with no separator yields "/":

getDirectory("README.md")   -> "/"     (expected "")
getDirectory("file.txt")    -> "/"     (expected "")
getDirectory("src/file.txt") -> "src/" (correct)

This is user-visible: the command palette renders directory + filename side by side (packages/app/src/components/dialog-command-palette-v2.tsx:282-283), so opening a repo-root file like README.md displays /README.md instead of README.md. Same pattern in dialog-select-file.tsx:135, context-items.tsx:27, and message-timeline.tsx:194.

Notably packages/app/src/pages/session/v2/session-file-list-v2.tsx:108 already works around it with value.includes("/") ? getDirectory(value) : undefined — the other call sites don't.

2. getFileExtension() returns the whole path when there is no extension. It splits on . and takes the last part, so with no dot the last part is the input:

getFileExtension("Makefile")       -> "Makefile"       (expected "")
getFileExtension("src/Dockerfile") -> "src/Dockerfile" (expected "")
getFileExtension(".gitignore")     -> "gitignore"      (expected "" — hidden file, not an extension)

It also ignores path structure, so getFileExtension("my.dir/Makefile") returns "dir/Makefile".

3. truncateMiddle() overflows instead of truncating at small widths. When maxLength is 1 or 2, Math.floor((maxLength - 1) / 2) is 0 and slice(-0) returns the entire string:

truncateMiddle("abcdefghij", 2) -> "a…abcdefghij" (12 chars, expected <= 2)
truncateMiddle("abcdefghij", 1) -> "…abcdefghij"  (11 chars, expected <= 1)

A truncation helper returning something longer than its input limit breaks any fixed-width layout that trusts it.

Steps to reproduce

import { getDirectory, getFileExtension, truncateMiddle } from "@opencode-ai/core/util/path"

console.log(getDirectory("README.md"))            // "/"
console.log(getFileExtension("Makefile"))         // "Makefile"
console.log(truncateMiddle("abcdefghij", 2))      // "a…abcdefghij"

OS

Windows (platform-independent — pure string logic)