Remove devcontainer support

[hbrodin]

Summary

• Remove devcontainer discovery and translation, the coop devcontainer command and related flags, remote repository lookup, and OCI Feature installation.
• Remove the JSONC parser and fuzz target; update CLI, lifecycle, integration tests, documentation, and release tooling.
• Remove obsolete feature-specific metadata through the existing private-storage migration.
• Preserve compatibility with generic state from existing environments and document the supported upgrade and recreation path.

Closes #525.

Verification

• cargo build --workspace
• cargo fmt -- --check
• cargo clippy --workspace --all-targets --all-features -- -D warnings
• cargo test --workspace (1,165 coop tests passed; 30 coop-proxy unit tests and 8 integration tests passed; one doc test ignored)
• cargo deny --workspace check, taplo format --check, and prek run
• cargo check -p coop --all-targets --target aarch64-apple-darwin
• Lifecycle mutation sweep: 99 mutants, 87 caught, 12 unviable, zero missed.
• Private-storage mutation sweep: 34 mutants, 30 caught, 4 unviable, zero missed or timed out.
• Fresh full macOS/Lima integration run after review fixes: 479 passed, 0 failed, 13 platform/environment/destructive-gate skips.
• Fresh full Linux/Firecracker integration run after review fixes: 488 passed, 0 failed, 9 platform/environment/destructive-gate skips.
• CLI rejects the removed command and flags; the integration suite confirms that a discovered project file is copied as ordinary workspace data without applying its environment or hook.
• Legacy metadata cleanup and compatibility behavior have focused regression coverage.
• Fresh review across correctness, design, conventions, security, tests, documentation, and comments completed with no remaining actionable findings.

[DarkaMaul]

I had a similar branch so I mostly asked Codex to do a diff between this branch and mine.

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The core removal looks sound, and the private-storage migration is a useful improvement: obsolete metadata is removed through checked directory descriptors without following symlinks. The integration coverage also verifies the important user-visible outcome—devcontainer.json remains ordinary workspace data while its environment and hook are ignored.
I found two compatibility/documentation gaps and one test-quality issue: preserve the --post-start reprovision caveat, pin deserialization of legacy template state containing oci_features, and make the removed-CLI test exhaustive and error-specific.
I reviewed correctness, design, conventions, security, tests, documentation, and changed comments. Focused CLI and migration tests passed. The full local suite encountered the same six environment-dependent failures in both compared implementations; I did not independently rerun the reported Lima and Firecracker integration suites.