chore: complete the Pluto to SpacePilot compatibility migration

[unfoundbox]

Context

PR #83 removes the deprecated public command, MCP aliases, and stale branded assets. Internal Pluto names and legacy readers remain because deleting them in the same change could strand local configuration, measurements, model caches, tokens, or cloud resources.

Scope

• rename the spacepilot.pluto package and pluto_mcp_server module, with a time-bounded import shim if released users require it
• choose stable user-data and user-config locations and migrate existing Pluto paths without data loss
• make SpacePilot environment variables canonical, retain read-only legacy fallbacks for a documented window, then remove them
• migrate X-Pluto-Token and pluto.localhost compatibility with an explicit deprecation schedule
• identify persisted IDs, logger names, salts, AWS names, and tags whose rename would change identity or create resources
• update tests, documentation, and shipped integrations after each compatibility boundary moves

Acceptance criteria

• a migration inventory classifies every remaining tracked Pluto reference as internal rename, persisted compatibility, external-resource migration, test fixture, or historical record
• upgrades preserve existing measurements, runtime state, model cache, and authentication
• migration is idempotent and covered by upgrade tests
• no new user-facing Pluto name is emitted
• all compatibility fallbacks have removal criteria and a target release

This should be phased rather than implemented as a blind repository-wide replacement.