fix: handle missing Instance context in Read tool

[sureshsankaran]

Summary

Fixes #8301

• Gracefully handle missing Instance context in Read tool and external-directory helper
• Prevents cryptic native binding errors when tools are executed outside normal server flow
• Falls back to path.basename() for title when Instance.worktree is unavailable

Problem

When the Read tool is executed outside of an Instance context (e.g., via MCP, plugins), accessing Instance.worktree throws a Context.NotFound exception. However, before this propagates, path.relative(undefined, filepath) throws:

TypeError: Binding expected string, TypedArray, boolean, number, bigint or null

This is confusing and does not indicate the actual problem.

Solution

Wrap Instance.worktree and Instance.containsPath() access in try-catch blocks:

• In read.ts: Fall back to path.basename(filepath) for the title
• In external-directory.ts: Proceed to ask for permission when containsPath check cannot run

Verification

• All existing read tool tests pass (26 tests)
• Tested fix logic with simulated missing context