Add a stable JSON CLI contract

[owainlewis]

Goal

Make Push commands reliable for automation and agent use without making human output worse.

Scope

• Add one documented global JSON output mode.
• Return JSON success payloads on stdout and structured JSON errors on stderr.
• Define stable exit-code categories for invalid input, configuration, unavailable dependencies, transient transport failures, conflicts, and unexpected failures.
• Add machine-readable output for doctor, status, resolved paths, job list, job show, job validate, and job runs.
• Keep the current concise human-readable output as the default.
• Ensure JSON mode never mixes logs, tables, or progress text into stdout.
• Include retryability only where Push can state it honestly. Unknown mutation outcomes must never be reported as safe to retry.
• Document schemas and shell examples suitable for jq.

Dependencies

• The resolved-path command should consume the runtime-path owner from Derive runtime storage from a single Push home #165 when available. The rest of the contract can be implemented independently.

Non-goals

• Do not require agents to send ordinary chat replies through the CLI.
• Do not add an HTTP server, daemon API, or UI.
• Do not expose credentials or message content in diagnostic payloads.

Acceptance criteria

• Every scoped command produces valid JSON with no unrelated stdout text when JSON mode is selected.
• Errors contain a stable category, human-readable message, and process exit code.
• Sensitive configuration values are redacted.
• Human output remains compatible unless a change is explicitly documented.
• Snapshot or integration tests cover success, validation failure, missing config, unavailable backend, and redaction.
• CLI reference and examples document the contract.

Verify

• cargo fmt --all -- --check
• cargo clippy --locked --all-targets -- -D warnings
• cargo build --locked
• cargo test --locked

[owainlewis]

Draft PR opened: #176

Verification on exact commit 93f554c, rebased onto current origin/main (3911f1b):

• fmt, clippy with warnings denied, and locked build pass
• full locked test suite passes: 408 tests
• strict MkDocs build and diff checks pass
• fresh independent final review approved with no remaining findings
• GitHub CI passes on Ubuntu, macOS, and the strict docs job
• coverage includes clean JSON streams, stable exit categories, validation and missing-config failures, unavailable dependencies, mutation rejection, credential/content/session redaction, PushPaths relocation, SQLite state isolation, and launchd/systemd status projections