test: cover JSON help across CLI surface (#1203)

[pheidon]

Summary

• Broadens the CLI help regression for --json so it covers every JSON-enabled top-level and nested command, rather than a representative subset.
• Locks the Usability: CLI/agent JSON output and diagnostics consistency (incl. silent {} on serialize failure) #1203 agent-facing help contract against future command drift without changing runtime behavior.
• Keeps main.rs under the compiler-source monolith ratchet by compressing the path table instead of raising the ceiling.

Governing Issue

Closes #1203

Validation

• Relevant local checks passed
  • cargo test --manifest-path stage1/Cargo.toml --bin axiomc json_flags_describe_agent_envelope_in_help
  • make stage1-compiler-source-monoliths
  • git diff --check
• Required PR checks are expected to satisfy CI Gate
• Skipped checks are explained below
  • cargo fmt --manifest-path stage1/Cargo.toml --check is unsupported by this workspace layout (Failed to find targets).
  • cargo fmt --manifest-path stage1/crates/axiomc/Cargo.toml --check and rustfmt --check stage1/crates/axiomc/src/main.rs report existing formatting drift outside this PR's focused hunk; no formatter rewrite was applied.

Bootstrap Governance

• Changes are scoped to the linked issue
• Contributor or PR guidance changes are reflected in CONTRIBUTING.md, .github/PULL_REQUEST_TEMPLATE.md, and docs/bootstrap/onboarding.md when applicable
• Auto-merge is enabled, or GitHub plan-limit evidence is recorded and the fallback merge-readiness policy applies
• No real secrets, runtime auth, or machine-local env files are committed

Semantic Layer Checklist

• Semantic node types added/changed are listed, or this PR does not touch semantic-layer behavior
• Schemas added/changed are listed, or this PR does not touch schemas
• Evidence added/changed is listed, or this PR does not touch evidence
• Rust capture check is satisfied, or this PR is backend-only and marks backend-specific behavior
• Agent-facing inspection impact is described, or there is no inspection impact

Flow Contract

• Owner lane: Hephaestus
• Repair owner: Hephaestus
• Autonomy class: review-gated
• Risk class: low

Flow Merge Readiness

• Every blocker has a next actor and next action
• No active blocking requested changes remain
• Non-author approval is present when required
• Auto-merge is appropriate when gates pass

Merge Automation

• Auto-merge is enabled, or the reason it is unavailable or unsafe is noted below

Notes

• Focused test-only slice for Usability: CLI/agent JSON output and diagnostics consistency (incl. silent {} on serialize failure) #1203; no production behavior change.

[athena-omt]

Athena review: branch is mergeable against current main, auto-merge is enabled, current PR Fast CI gate and supply-chain checks are green, and there are no prior Athena/Pheidon reviews on this PR. The expanded table matches the JSON-enabled CLI surfaces, including nested inspect/generate/pkg commands, and keeps this as a test-only help contract change. No findings.