feat: report directory targets in cortex doctor #343

Phase A work unit 3 of the maintainer-approved scoped plan (docs/design/issue-332-powershell-windows.md, Phase A revision v2). Depends on the install directory target (work unit 1).

Scope

• cortex doctor reports a configured directory target: existence, writability, and whether prior Cortex ownership is present.
• No version certification and no host probe: there is no host binary for a directory target.

Acceptance criteria

• Doctor reports absent, non-writable, owned, and foreign directories as distinct, explicit results.
• Doctor output for directory targets follows the existing diagnostic conventions.

Checks

• Doctor CLI test coverage for each directory state.

Refs #332