docs: volume help says the attach default is 10Gi

[tonychang04]

Draft — gated on insta-platform#327 reaching production.

Two --help strings quoted the old 1 GiB attach default:

• insta services add --volume <gi> — "any plan may attach at the default 1" → 10
• insta compute volume — "any plan at the default 1Gi" → 10Gi

Nothing else in the CLI hardcodes the number: computeVolume sends only what --size gives and prints cap.volumeGib straight from the API, so its output was already correct.

npm run typecheck clean; vitest run 672 passed / 47 files.

🤖 Generated with Claude Code

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Summary by cubic

Updates the --help text in two places to say the default volume attach size is 10 GiB instead of 1 GiB, matching the new free volume cap.

• The changed strings cover insta services add --volume and insta compute volume.
• No other CLI code hardcodes the old number; computeVolume output already comes from the API.

^{Written for commit 059a636. Summary will update on new commits.}