fix: persist special inode dentries in sqlite

[NathanFlurry]

• Add a forward SQLite migration for character, block, and FIFO dentry kinds while preserving existing rows.
• Cover schema upgrades and persistent special-node metadata through the chunked-local production path.
• Keep dormant ObjectS3 integration bodies intact and out of the default test suite until return-to-service.

Validation: cargo fmt --all -- --check, cargo check --workspace, cargo test -p agentos-vfs, and strict pinned generic/062 across all supported backends.

[railway-app]

🚅 Deployed to the agentos-pr-1775 environment in agentos

Service	Status	Web	Updated (UTC)
agentos	😴 Sleeping (View Logs)	Web	Jul 18, 2026 at 4:27 pm

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🚅 Environment agentos-pr-1775 in rivet-frontend has no services deployed.