Eliminate statSync calls and filter prefixes first in path completion

Summary

• In cli/src/utils/path-completion.ts, optimize getPathCompletion to eliminate $O(N)$ synchronous disk statSync calls during CLI tab completion.
• Previously, getPathCompletion called readdirSync(parentDir) and immediately ran statSync(fullPath).isDirectory() on every single item in the directory before checking if it matched the typed prefix. In directories with thousands of files (e.g. user home, desktop, monorepo roots), this executed thousands of synchronous filesystem syscalls on every Tab press, freezing the TUI.
• Switched to readdirSync(parentDir, { withFileTypes: true }) and placed the prefix match check (!name.toLowerCase().startsWith(partial)) first. For matching items, entry.isDirectory() checks inode type directly with zero extra syscalls, only falling back to statSync when a symbolic link needs resolution.
• Benchmark: In a directory of 2,500 files, reduced statSync syscalls from 2,505 per Tab completion down to 0, improving completion latency by 6.2x (102.56ms -> 16.55ms across 20 iterations).
• Adds regression unit test in cli/src/__tests__/path-completion.test.ts verifying that symlinked directories continue to resolve and complete correctly.

Test plan

[✓] bun test --config=/dev/null src/__tests__/path-completion.test.ts — 25 pass, 0 fail
[✓] bun test --config=/dev/null src/hooks/__tests__/use-path-tab-completion.test.ts — 33 pass, 0 fail
[✓] bun run --cwd cli typecheck — 0 errors
[✓] PR hygiene check passed