feat(common): add SavingsPlanDetails.Currency

[cristim]

Refs #46

Pkg-only step 1 of 2: adds SavingsPlanDetails.Currency (additive, no behaviour change) so the Azure change can follow with pins bumped to this merge commit. Step 2 (Azure helper, three call sites, tests, pin bumps) carries Closes #46.

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• CHANGELOG.md
• pkg/common/types.go

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[cristim]

Merge-gate verdict (independent review, claude-opus-5-5): APPROVE at 0b360d91f1957ca2d03bf28dec0c3623301399c5

Scope: diff is only SavingsPlanDetails.Currency + godoc (pkg/common/types.go, +5/-1, the -1 is gofmt realignment of Coverage) and one CHANGELOG entry. Nothing else.

Checks:

• Field/tag: Currency string json:"currency,omitempty", snake_case like siblings; same name and tag as fix(azure): require explicit currency on Savings Plan commitments #344 uses. omitempty: older persisted Details JSON decodes to "", and an empty value serializes byte-identically to before, so JSON round-trips and any hashes over serialized Details are unchanged.
• Additive across all four repos: AST scan (go/parser, every SavingsPlanDetails composite literal) over this tree plus cloud-commitments-cli, -mcp, -platform checkouts: 77 literals, 0 unkeyed.
• No behaviour change: no provider code reads the field at this SHA (follows the feat(common): export ErrOutcomeUnknown sentinel for lost-response purchases #329 then refactor(aws): use common.ErrOutcomeUnknown and drop the internal sentinel #335 precedent). Local at exact head (git archive, go1.26.9, GOMAXPROCS=2, -p 2): go test ./... exit 0 for pkg, providers/aws, providers/azure, providers/gcp (workspace, i.e. providers built against this pkg); pkg with GOWORK=off exit 0; go vet pkg exit 0.
• git secrets --scan over all tracked files: clean. CI: all checks green at this SHA, mergeStateStatus CLEAN, branch contains origin/main. Labels mirror fix(azure): Savings Plan purchase hardcodes CurrencyCode USD #46; body says Refs #46.

Non-blocking note: the godoc's "fails closed when it is empty or malformed" describes the Azure client after step 2 (#344); at this SHA the Azure client still hard-codes USD. The CHANGELOG states the field is unused until then, so this is accurate once #344 lands. No mutation sweep (tiny additive change, per policy).