fix(rates): treat a bare flat fee as a project fee, not per-hour

[clawedassistant26]

The bug

parseRate() in src/rates.mjs has a guard whose comment states the intent plainly:

// A flat fee with no period stated is a project fee, not an hourly one: "$5000
// for the project" is how it is written, and defaulting it to per-hour would
// silently multiply the invoice by every hour tracked.
if (!sawPeriod && rate.unit === "flat" && rate.cap === null) rate.per = "hour";

The assignment set rate.per = "hour" — the exact behaviour the comment warns against. Since the default is already "hour", the guard was a no-op, so a bare flat fee like $5000 or 250 USDC parsed as an hourly rate.

chargeFor() then scales an hourly rate by tracked time, so a $5000 fee tracked over a 10-hour job bills $50,000:

parseRate("$5000")                        => { per: "hour", ... }
chargeFor({ seconds: 36000 }, that)       => { amount: 50000, flat: false }

A project fee is meant to be flat (amount: null, added once by the invoice):

parseRate("$5000/project")                => { per: "project", ... }
chargeFor({ seconds: 36000 }, that)       => { amount: null, flat: true }

The fix

One line: set rate.per = "project" in that guard, matching the documented intent. A stated period ($100/hour) is untouched, and $100/agent (a unit but no period) still defaults to hourly as before.

Test

Added a regression test in test/rates.test.mjs asserting a bare fee (fiat and crypto) parses as a project fee, bills flat over a 10-hour job, and that an explicit period is left alone. Verify-first: the test fails on the pre-fix line and passes with the fix. Full suite green (2375 pass / 0 fail / 337 skip).