Simplified manager contracts, recovery state and override inventory

[acburdine]

ref https://linear.app/ghost/issue/PLA-511/simplify-and-harden-the-next-docker-manager-following-branch-review

The documentation mixed current contracts with completed work and superseded designs, while manager code maintained duplicate override inventories and more update stages than recovery needed. This pass consolidates those sources without changing the recovery boundary or supported workflows.

Changes

• Give current invariants an authoritative home in docs/architecture.md; keep operator usage/recovery in installation docs, configuration contracts in configuration docs, and outstanding requirements in the roadmap. Schemas and file inventories remain authoritative in code.
• Remove implementation chronology, completed-work reports, stale claims and repetitive comments. Reduce AGENTS.md to routing, execution constraints and high-value warnings.
• Derive additional overrides from Compose's ordered file list and share the rule between backup and restore. With GD_COMPOSE_OVERRIDES=overrides/compose.override.yml and no root override, the custom file is now captured and restored. Tests cover both root-override states and real Compose merging with relative/absolute paths.
• Reduce update recovery state to whether service startup was attempted, retaining newer writes after that boundary. Remove unused context and repeated transformations.

Based on 5a392166, the merged result of #380. Its nested-override regression was still present; no follow-up fix was found. Compose ownership, image-only self-update, migration version requirements, exact-version import, native bulk-data tools and remaining roadmap requirements are preserved. No dependencies or compatibility layers were added.

Validation

• 469 unit tests and 26 real-Compose integration tests passed, with no skips.
• 57 final CLI/config checks passed; formatting, lint, types, ShellCheck, local documentation links and git diff --check passed.
• Image-update E2E scenarios passed, including writes retained after failed startup and explicit operator recovery.
• The combined self-update E2E failed at checkout restore with a Git-discovery error. The isolated checkout scenario subsequently passed on both the unchanged base and this branch, including wrong-commit refusal and successful restore. The combined-run failure remains unexplained; the full suite is not claimed green.

Further removal of recorded installation metadata would change what info promises and needs a product decision; that contract is unchanged.

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