feat: project capabilities to a user-selected directory target on install #341

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

Scope

• cortex install --target-dir <path> projects skills and QA actors as plain SKILL.md files into the chosen directory, inside the existing install transaction (manifest, ownership, backup, rollback, readback).
• No new RuntimeID family and no internal/runtimematrix entry: this is a destination mode, OS-agnostic, usable on POSIX today.
• The Agent Skills specification (agentskills.io / Microsoft Agent Framework) is the consumption contract for the output; Cortex's metadata.cortex-* frontmatter passes through. Document this contract where the mode is documented.

Acceptance criteria

• Install into an empty target directory writes the full capability set with a manifest that records provenance and hashes.
• A failed write mid-install leaves the directory unchanged (transactional, all-or-nothing).
• Readback verifies what was written; ownership records what Cortex owns.
• Refusing or failing paths (nonexistent, unwritable) fail closed with an explicit diagnostic.

Checks

• Isolated install/readback against a temp directory; rollback path exercised; manifest hash verification.

Refs #332