SMOODEV-2259: th api agents mint/update --extension for per-agent SEP extensions

[brentrager]

Problem

Per-agent SEP extension enablement (SMOODEV-2259) needs a way to set an agent's extensionConfig from the CLI. th api agents mint/update already carry toolConfig via --tool-config, but there was no equivalent for extensionConfig.

Solution

Add a --extension flag to both th api agents mint and th api agents update that carries the agent's extensionConfig JSON into the create/update request body, mirroring the existing --tool-config flag exactly:

• Shape: {"enabledExtensions":[{"extensionId":"plan-mode","enabled":true,"config":{}}]}
• @-prefix reads the JSON from a file; non-object input fails loudly (parse_json_object_flag).
• extensionId is kebab-case (SEP extension name). Empty enabledExtensions = no extensions (fail-closed).
• Threaded through build_mint_body / build_update_body (both pure, unit-tested) → extensionConfig key in the body.
• Added to the update has_flags guard and the "nothing to update" bail message.

Verification

• cargo test -p smooai-smooth-cli agents — all agents tests pass (17).
• cargo fmt clean; cargo clippy --workspace (pre-commit hook) passes (only pre-existing pedantic/nursery warnings, none from this change).
• Extended mint_body_carries_per_agent_config and update_body_maps_every_field to pass an extension payload and assert body["extensionConfig"].
• Docs: docs/Engineering/Using-th-CLI.md agents section documents --extension beside --tool-config.

Part of SMOODEV-2259 (per-agent SEP extension enablement). Mirrors the SMOODEV-590 typed-flag pattern.

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Latest commit: 9519700

The changes in this PR will be included in the next version bump.

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@smooai/smooth	Minor

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