feat(zpm): add ZPM devcontainer feature

[pocky]

Summary

• Add a new zpm devcontainer feature that installs ZPM, the Zig-based MCP server exposing a Trealla Prolog logic engine over STDIO for deterministic logical reasoning
• The feature resolves the latest version from GitHub releases or accepts a pinned version, detects architecture (amd64/arm64), downloads the pre-built binary, and verifies its SHA256 checksum before installing to /usr/local/bin/
• Includes a full test suite with two scenarios: default (latest) install and version-pinned install (v0.3.0)
• Documents the new feature in the README with usage examples, option reference, and architecture support notes

Changes

Feature

• src/zpm/devcontainer-feature.json: Feature metadata with version option (default latest), documentation and license URLs
• src/zpm/install.sh: Installation script with architecture detection, GitHub API version resolution, binary download, SHA256 checksum verification, and install to /usr/local/bin/zpm

Tests

• test/zpm/scenarios.json: Two test scenarios — install_zpm_latest and install_zpm_specific_version (v0.3.0)
• test/zpm/test.sh: Base assertions — binary on PATH, zpm version returns non-empty output
• test/zpm/install_zpm_latest.sh: Scenario script sourcing test.sh for the default install
• test/zpm/install_zpm_specific_version.sh: Scenario script verifying the installed version matches the pinned 0.3.0

Documentation

• README.md: Add ZPM to the table of contents, add feature section with devcontainer.json usage snippets, options table, version-pinning example, and architecture support note; update repository structure tree

Test plan

• Run ./test-local.sh zpm — both scenarios must pass
• Verify zpm version returns a non-empty string inside the container
• Pin version: v0.3.0 and confirm the installed version matches exactly
• Confirm ShellCheck passes on src/zpm/install.sh with no errors

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