feat(cli): add json output for policy get

[xmh1011]

Summary

Adds JSON output support to openshell policy get so users can inspect policy revision metadata and full policy payloads with tools like jq.

Related Issue

Fixes #1034

Changes

• Added --json to openshell policy get.
• Added JSON rendering for sandbox-scoped and global policy revisions.
• Includes .revision.policy only when --full is passed.
• Added CLI parsing and JSON-shape unit tests.
• Updated policy workflow docs with a jq example.

Testing

• mise run pre-commit passes
• Unit tests added/updated
• E2E tests added/updated (if applicable)

Ran:

• cargo fmt --all -- --check
• cargo test -p openshell-cli policy_get_accepts_json --bin openshell
• cargo test -p openshell-cli policy_get_json --lib
• cargo clippy -p openshell-cli --all-targets -- -D warnings
• git diff --check

Note: mise run pre-commit could not be run locally because mise is not installed in this environment.

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture docs updated (if applicable)

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[github-actions]

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[xmh1011]

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[xmh1011]

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