docs(agent): mark spark-1 models deprecated in help text and README

[mogery]

Part of removing the spark-1 agent models so spark-2 is the only generation left.

• The --model help text and the AgentOptions.model doc comment now say spark-2 is the default and spark-1-mini / spark-1-pro are deprecated and run spark-2.
• The README options table no longer calls spark-1-mini the default. The "higher accuracy" example now uses --effort high instead of --model spark-1-pro.

Kept on purpose: --model spark-1-pro / spark-1-mini are still accepted by the local validation and the AgentModel type. The API accepts them as deprecated aliases that run spark-2, so scripts that pass them keep working instead of failing locally.

pnpm test gives the same 41 failures on main without this change (Alexandria discovery tests), so they're unrelated.

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Summary by cubic

Marks the spark-1 agent models as deprecated in the CLI help text, AgentOptions doc comment, and README so spark-2 is presented as the only current model. The README options table now documents the --effort option, and the higher-accuracy example uses --effort high instead of --model spark-1-pro.

spark-1-mini and spark-1-pro are still accepted as deprecated aliases that run spark-2, so existing scripts that pass them keep working.

^{Written for commit 75ca31b. Summary will update on new commits.}

[cubic-dev-ai]

All reported issues were addressed across 3 files

<sub>Shadow auto-approve: would auto-approve with 1 open P3 issue. Docs-only update marking spark-1 agent models deprecated in CLI help, AgentOptions comment, and README, and switching the README example to --effort high. No behavior change: aliases still accepted, so the change is bounded and clearly beneficial.

Fix all with cubic | Re-trigger cubic</sub>

[cubic-dev-ai]

0 issues found across 1 file (changes from recent commits).

Confidence score: 5/5

• Automated review surfaced no issues in the provided summaries.
• No files require special attention.

<sub>Shadow auto-approve: would auto-approve. Docs-only update: CLI help text, AgentOptions comment, and README now present spark-2 as default with spark-1 aliases deprecated, and the README example uses --effort high. No behavior code changes, so risk is limited to documentation accuracy.

Re-trigger cubic</sub>