chore: drop migrations every active install has already run

[stefanhoelzl]

PostHog shows every install active in the last 30 days on 2026.9.27 or newer, so the one-shot conversions shipped up to that release have run everywhere they still matter.

• Remove the auto-workspace.sources → plugin move (legacy-sources.ts); the automations tests now use plain item templates as fixtures
• Remove the config.json move from the data root to ~/.codehydra (Config.moveLegacyConfig)
• Remove the Windows %APPDATA% → %LOCALAPPDATA% data-root relocation (data-root-relocation.ts, PathProvider platformRoot) and its Documented Exceptions row
• Remove the startup rewrite of legacy {path, remoteUrl} managed-project records and the matching "Prepare project records" step of a workspaces move; loadAllProjects now returns plain configs
• Remove the startup sweeps of claude/configs, code-server/ and opencode.codehydra.json (the retire rule kind stays)
• Remove the hooks.trusted fallback for repository plugin trust (an old project asks once)
• Remove the .codehydra/hooks Migrate offer (legacy-hooks.ts, editor warning, e2e scenario, guide section); old hook files are now simply ignored
• Remove the hooks.enabled, automations.poll-interval and auto-workspace.poll-interval aliases (no active install sets them)
• legacyNames stays as the rename mechanism but is now one-way: Config.load() moves a value found under an old name to the new key in config.json and drops the old entry (no downgrade support), so an alias can go one release after a rename
• USER_GUIDE, ARCHITECTURE and CLAUDE.md updated to match

Migrations added after 2026.9.27 (adoption conversion, metadata off git config, ref-keyed state, workspace name fallback, paths.workspaces-current takeover) stay until the last 2026.9.27 install updates.

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