✨ Add --model CLI flag and ASKCC_CLAUDE_MODEL env var (closes #110)

[monkut]

Summary

• Adds the --model CLI flag and ASKCC_CLAUDE_MODEL env var, completing the precedence chain symmetry started in ✨ Add subagent frontmatter to agent action definitions #84
• New _resolve_model helper in askcc/cli.py mirrors _resolve_effort: CLI > env > frontmatter > unset (returns None, no flag emitted)
• Runner.run gains a model: str | None kwarg; _frontmatter_cli_flags accepts an override and falls back to config.model

Behavior

• askcc --model sonnet develop <url> overrides model: opus in DEVELOP_SYSTEM_PROMPT.md
• ASKCC_CLAUDE_MODEL=sonnet askcc develop <url> does the same when no --model flag is set
• --model opuz rejected by argparse (exit 2); ASKCC_CLAUDE_MODEL=opuz logs a warning and falls through
• Unset everywhere → no --model flag emitted, claude picks its own default (unchanged behavior)

Tests

• TestModelPrecedence — CLI > env > frontmatter > unset (None propagates)
• TestModelCLIFlags — CLI flag and env var both reach runner.run(model=...); invalid CLI value rejected by argparse
• TestResolveModel — unit tests for the helper including warn-and-ignore on invalid env var
• TestRunnerFrontmatterFlags extensions — runner-level override and config fallback

Test plan

• uv run pytest — 272 passed
• uv run ruff check — all checks passed
• uv run pyright — 0 errors

Closes #110