docs(README): document per-account service overrides (closes #117)

[cristim]

Summary

Closes #117.

Per-account service overrides have been a feature since migration 000011_cloud_accounts.up.sql (mid-2024) and now have full UI coverage thanks to #72 (inline payment edit) and #106 (create modal). None of it was documented, so users landing on the dashboard saw the panel + the Add modal with no narrative for what overrides ARE, when to use them, or how they interact with the global defaults.

What's added

A new "Per-Account Service Overrides" section between "Coverage Percentage" and "Safety Features" covering:

• Concept + when to use vs the global Settings → Purchasing card (with examples: dev/staging carve-outs, workload-shape outliers, pilot rollouts).
• Web-UI walkthrough: Settings → Accounts → expand → Service overrides → modal.
• AWS-only V1 boundary callout pointing at ux(settings): create-override modal for Azure and GCP accounts (V2 of #106) #109 for Azure/GCP UI.
• How to edit existing overrides: inline Payment per ux(settings): add payment option selector to AWS account overrides modal #72, Reset deletes, ux(settings): inline edit Term/Coverage + Enabled toggle on existing override rows (V2 of #72) #110 widens to Term/Coverage/Enabled.
• "Inherit" semantics: blank fields are NOT stored as a sentinel — the PUT stays sparse; the engine reads the global default at evaluation time, so changes to global defaults flow through automatically.
• API parity note: the modal targets the same endpoint as scripted setups; both write the same row.

.markdownlint.yaml

Disables MD060 (table-column-style) — same rationale as PR #169. Landing here too in case the PRs merge in a different order; both diffs are idempotent.

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[cristim]

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• README.md

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch docs/issue-117-account-overrides

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[cristim]

Rebased onto current feat/multicloud-web-frontend (was c2c7a5e, now abb5d35). Conflict was in .markdownlint.yaml only — README.md merged cleanly. Same conflict pattern as PR #169: upstream PR #105 pinned MD060: leading_and_trailing; this PR's commit additionally disabled MD060. Resolved by keeping upstream's pin and dropping the PR's MD060 change — the commit message already noted both diffs are idempotent across landing order. Verified markdownlint README.md is clean against upstream's pin. @coderabbitai review

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@cristim Sure! Triggering a review of the rebased changes now.

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