docs: fix a/an grammar in hook wrapper docstring

[chayu163]

Summary

• Fixes a grammar slip in the Claude Code hook wrapper docstring: a agentacct → an agentacct (src/agentacct/hooks.py:1126).

Fixes #198

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

• Docstring-only change
• Public docs avoid private strategy/roadmap claims