fix(cli): require explicit MCP registration replacement

[sizzlecar]

Re-running orchestral mcp add with an existing name previously replaced its entire registration, including environment and permission settings, before printing Updated. It now fails without writing the registry unless --replace is explicit. Replacement builds a new entry from the supplied arguments and existing defaults; the help explains that omitted settings reset.

For example, orchestral mcp add my-tool -- new-command preserves an existing my-tool entry and reports the duplicate. orchestral mcp add my-tool --replace -- new-command explicitly replaces it.

The real CLI regression verifies byte-for-byte preservation after rejection, replacement of command/environment/permissions and other settings, exclusion of inherited Host environment from the MCP entry, and preservation of other servers. Added an Unreleased changelog entry; the version remains unchanged.

Validation:

• cargo test --locked -p orchestral-cli --test agent_live_e2e mcp_user_registry_ -- --nocapture: 2 passed, 0 failed, 0 ignored; 80 unrelated tests filtered out.
• cargo fmt --all -- --check and git diff --check: passed.
• Built CLI mcp add --help: checked the replacement/reset explanation.

The full workspace and platform checks are left to the existing PR CI; no model calls or user registry were used locally.