F2.1 (reshape): Source-abstract fleet model + local-kubeconfig source adapter

[gnanirahulnutakki]

Part of epic #20 (E2). Milestone: Phase L (day-0 adoption wedge). Source: docs/EPICS.md / docs/SITH-NOTION.md.

Make the E2 fleet-model source abstract so the same read / model / correlation code powers both the day-0 local mode (source = the user's local kubeconfig contexts, client-side fan-out) and the day-N hub (source = an OCM-brokered spoke). This is the reshape delta on F2.1 and the engine the local wedge (E11) needs.

Scope

• A common FleetSource interface with two adapters: local-kubeconfig (this issue's focus) and OCM-spoke (generalizes the existing spoke work in P1: Read-federation service — normalized fleet model across 2 spokes #9).
• The local adapter reads kubeconfig contexts, honors exec-credential plugins locally, and streams inventory/health into the same normalized fleet model (freshness + source stamped).
• Everything above the source — the store (P1: Read-federation service — normalized fleet model across 2 spokes #9 / F2.2), correlation (P1: Cross-cluster correlation query #10 / F2.3), staleness — is unchanged and shared.

Acceptance

• The fleet model is populated from local kubeconfig contexts with no hub/OCM.
• The same model code serves an OCM-spoke source unchanged.
• One correlation query (P1: Cross-cluster correlation query #10) returns a correct answer over ≥ 2 local contexts.

Related: #20 (E2), #29 (E11), #9, #10.

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Spec & slice. Slice 1 of docs/BUILD-SEQUENCE.md (PR #40). Full contract: docs/specs/F2.1-source-adapter-contract.md (PR #41) — the source-abstract 7-verb connector/source-adapter seam. Depends on Slice 0 (#47). Co-lands with F11.1 (#32). Slice-1 AC (BUILD-SEQUENCE §Slice 1): all contexts detected; unreachable surfaced never fatal; exec-plugins run locally; every record observed_at+source stamped; a second in-memory adapter satisfies fleet.Source unchanged (interface-parity test).