feat: reserve telemetry webhook and job-summary fields

[joshua-temple]

Problem

The v1 manifest schema is about to freeze. The reserved vendor-neutral telemetry
seam (config.telemetry / TelemetryConfig) currently reserves only enabled
and adapter. Issue #3 asks for a richer metrics surface (a sink endpoint and a
run-summary toggle). Adding a separate metrics: block would duplicate that seam
and create two parallel observability surfaces, so the enrichment converges onto
the existing telemetry block instead.

Fix

Shape-only reservation, additive and omitempty, no generation/emit behavior and
no schema_version bump:

• TelemetryConfig gains webhook *TelemetryWebhook (a generic vendor-neutral
  JSON-POST sink) and job_summary *bool (a *bool so unset stays distinct from
  an explicit false, mirroring the fail_fast precedent).
• New TelemetryWebhook type with url and secret_name. secret_name is the
  name of a GitHub Actions secret holding the auth token, a reference and never an
  inline credential.
• Adapter value constants (none, datadog) for documentation and future use.
  The vendor stays a value behind the adapter seam; no vendor client is wired into
  core.
• Lenient validation only on the newly reserved fields (webhook url must be
  http(s) when set; secret_name must be a safe GHA secret name). adapter is
  left unchecked so an arbitrary value that parses today keeps parsing.
• JSON Schema (all three synced copies), versioning docs, and an e2e scenario.

No metrics: key is added; the reservation lives entirely on telemetry.

Verification

• go build ./cmd/... ./internal/..., go test ./... (1501 passed),
  golangci-lint run ./... (clean).
• e2e module go build ./..., go vet ./..., golangci-lint run ./... (clean).
• Generator byte-identity test proves a populated telemetry block produces a
  byte-identical plan set to one that omits it (hardened against a vacuous pass).
• Parse/round-trip and schema-version tests assert the new fields validate at the
  current schema_version without a bump.
• All three schema copies (internal/schema, schema/, docs/public) regenerated
  via cascade schema and confirmed byte-identical.

Closes #3