fix(scripts): align subscriber-health-dashboard output with SubscriptionHealth schema and define aggregate exit codes

[AGWAM001]

Closes #892

Summary

Aligns subscriber-health-dashboard.ts's JSON output field names one-for-one with the contract's SubscriptionHealth struct (active, charge_due, within_grace, has_sufficient_allowance, is_paused, trial_active, daily_limit_set), adds address identity to output, defines aggregate exit-code semantics (0 = all healthy, 1 = any unhealthy, 2 = hard failure), and adds fixture-driven tests including at least one unhealthy case.

Problem

subscriber-health-dashboard.ts (ops CLI) and the frontend SubscriptionHealthWidget both interpret the same contract SubscriptionHealth struct, but the CLI's field naming has drifted from the struct's actual field names. Combined with weak/undefined exit-code semantics, this makes CLI output hard to directly compare against the UI and unreliable to consume in automation (scripts can't cleanly branch on "healthy vs. unhealthy vs. broke").

Changes
Schema alignment: CLI JSON output now emits fields matching SubscriptionHealth exactly — active, charge_due, within_grace, has_sufficient_allowance, is_paused, trial_active, daily_limit_set — plus subscriber address identity included alongside each entry.
Same source of truth as UI: CLI reads the same contract struct fields via RPC that SubscriptionHealthWidget reads, rather than deriving/renaming a parallel representation — eliminating the drift at the source rather than just patching field names.
Exit-code semantics: defined and implemented aggregate exit codes:
0 — all checked subscribers healthy
1 — at least one subscriber unhealthy (but the check itself ran successfully)
2 — hard failure (e.g. RPC error, malformed response, couldn't complete the check)
--json output: JSON output (via --json flag or as default, suitable for piping) matches the aligned schema.
Fixtures: added fixtures under scripts/data/ covering healthy and unhealthy subscriber states, including at least one fixture asserting the aggregate exits 1 (unhealthy).
README: added a sample output block in scripts/README.md showing the JSON schema matching SubscriptionHealth field names, and documented the exit-code meanings.
Explicitly out of scope
No web server / hosted dashboard — this remains a CLI/script tool, per the issue's stated scope.
Why this approach

Reading the same contract fields the UI reads (rather than maintaining a separately-named CLI schema) removes the drift at its root — any future SubscriptionHealth field change surfaces identically in both places instead of requiring two separate updates that can fall out of sync again. Three-way exit codes (healthy / unhealthy / hard failure) let automation distinguish "the check ran and found a problem" from "the check itself couldn't run" — collapsing those into a single non-zero exit would make monitoring alerts ambiguous.

Testing / Validation
Fixture-driven test run confirms JSON keys match SubscriptionHealth field names exactly (including address identity).
Fixture covering an unhealthy subscriber asserts aggregate exit code 1.
Fixture covering a simulated hard failure (e.g. malformed/failed RPC response) asserts exit code 2.
Fixture covering all-healthy subscribers asserts exit code 0.
README sample output verified to match the actual schema emitted by the script (not hand-written/aspirational).

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