fix: avoid array spread when merging messages

[stephanschuler]

What & why

mu aborts with ❌ Error fetching usage data: Maximum call stack size exceeded and prints
nothing once a source returns more than roughly 100k messages. My ~/.claude/projects/ holds
120,654 assistant messages, which is enough to hit it on every run — with and without -s,
and -t / -p are no workaround because filtering happens after the merge.

The three merge sites append with a spread, so every message becomes its own argument to
Array.prototype.push and V8 throws past its argument limit:

• src/usage.ts:76 — merges all sources, the one that actually fails today
• src/sources/claude.ts:64 and src/sources/pi.ts:64 — still below the threshold because
  they accumulate per directory, but the same failure waits there

Closes #9

How

Each site pushes in a loop instead. One comment per site names the trap, so the spread does
not come back as a "simplification" later:

// no spread: RangeError past ~100k args
for (const m of messages) allMessages.push(m);

No behaviour change beyond not crashing: same messages, same order, same totals. Fixing only
usage.ts would have left two loaded guns in the adapters, which is why all three are in
here — it is the same one-line change three times, not three concerns.

Verification

• npm run build passes
• Verified the change by running node dist/cli.js with the affected flags
• Docs updated if behavior changed (README.md, AGENTS.md, and the help text in
  src/cli.ts stay in sync)

Details:

$ npm run build          # tsc, clean

$ node dist/cli.js -s claude
  Results: 3 entries (149 msgs from 120675 total)   # was: RangeError

$ node dist/cli.js       # all sources, no filter
  Results: 3 entries (150 msgs from 120676 total)   # was: RangeError

Docs need no change — the observable behaviour is unchanged, the command simply stops
crashing. No new flags, no new output, so README.md, AGENTS.md and the help text in
src/cli.ts stay as they are.

No tests: the repo has no test setup, and a regression test for this would have to build
100k+ objects to trip the limit. Happy to add one if you want the infrastructure.