cli: add --output file option to receipt --markdown

[chayu163]

Summary

• Adds --output <path> to receipt --markdown (src/agentacct/cli.py:9866), writing the rendered Markdown receipt to a file instead of stdout.

Fixes #200

Test plan

• pytest -q
• Secret scan
• No paid provider calls

Safety checklist

• Preserves local-first behavior
• Does not store API keys or secrets
• Does not expose localhost services publicly by default
• Does not control processes agentacct did not start
• Provider/API forwarding remains opt-in and budget-gated
• JSON output paths remain machine-readable when --json is used

Docs

• Help text only — no README change
• Public docs avoid private strategy/roadmap claims