docs: clarify /metrics is JSON, not Prometheus

[yesyayen]

What

Correct the documentation to match reality: /metrics returns JSON, not Prometheus exposition format.

Changes:

• Update README, AGENTS.md, 5 design docs, and 3 manuals to describe the actual JSON schema and the Amazon DynamoDB CloudWatch-style metric names and dimensions it uses.
• Drop the unused metrics-exporter-prometheus dependency.

Why

Wiring local Grafana against ExtendDB, I expected Prometheus exposition format and got JSON. Match docs with whats implemented.

Closes # n/a

Testing done

Live /metrics capture against a local server:

$ curl -ksS -i 'https://127.0.0.1:8000/metrics?window=LastDay'
HTTP/2 200
content-type: application/json

Trimmed real response (field names and structure unmodified):

{
  "metrics": [
    {
      "metric": "SuccessfulRequestLatency",
      "dimensions": [{ "TableName": "abac-prod-c73b90ce" }, { "Operation": "CreateTable" }],
      "window": "LastDay",
      "sum": 11521.0, "count": 1, "min": 11521.0, "max": 11521.0
    },
    {
      "metric": "UserErrors",
      "dimensions": [{ "TableName": "extenddb-ie-test-cc0e7284" }, { "Operation": "DescribeTable" }],
      "window": "LastDay",
      "sum": 1.0, "count": 1, "min": 1.0, "max": 1.0
    },
.
.
.
  ],
  "source": "database"
}

This is structured JSON with typed dimension keys, not line-oriented text with # HELP / # TYPE headers, so a Prometheus scraper cannot consume it.

Checklist

• I have read CONTRIBUTING.md
• All tests pass (cargo test --workspace) (no Rust source changed)
• Code is formatted (cargo fmt --check) (no Rust source changed)
• Clippy is clean (cargo clippy -- -W clippy::pedantic) (no Rust source changed)
• I have added or updated tests for new functionality (n/a, documentation)
• I have updated documentation if behavior changed
• Breaking changes are noted below (none)
• If this changes the wire protocol, Storage trait, auth model, on-disk format, or public CLI surface, an RFC has been accepted or is linked below. Otherwise, an ADR captures the decision. (n/a, documentation)

ADR / RFC: n/a

Breaking changes

None.

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