fix: send OpenRouter app attribution headers on compaction requests

[KD-MM2]

Problem

Compaction routed to OpenRouter free models fails(somes, not all) with a 403(). OpenRouter
only serves its free endpoints to registered agentic harnesses, identified by
app-attribution headers (HTTP-Referer, X-OpenRouter-Title,
X-OpenRouter-Categories). Pi adds these headers to session requests via its
provider-attribution module, but the exported compact() path bypasses that
pipeline(so requests made by this extension never carried the attribution
and were rejected).

Fix

The extension now adds the same attribution headers Pi sends, with the same
gating semantics:

• withOpenRouterAttribution() (src/index.ts) merges
  HTTP-Referer: https://pi.dev, X-OpenRouter-Title: pi, and
  X-OpenRouter-Categories: cli-agent into the compaction request headers,
  only for OpenRouter models (provider openrouter, or a base URL on
  openrouter.ai). Caller-provided auth headers take precedence, matching
  Pi's mergeProviderAttributionHeaders merge order.
• isInstallTelemetryEnabled() (src/config.ts) mirrors Pi's gate:
  attribution is only sent when install telemetry is enabled
  (enableInstallTelemetry setting, overridable via PI_TELEMETRY with
  1/true/yes).
• createSettings() (src/config.ts) extracts SettingsManager creation so
  the hook builds one manager shared by config loading and the telemetry
  check.

Tests

• New test/headers.test.ts (5 tests): attribution added for OpenRouter,
  baseUrl-based detection for custom providers, non-OpenRouter models
  untouched, telemetry-disabled untouched, caller-header precedence.
• test/config.test.ts (+3 tests): telemetry gate settings fallback and
  truthy/falsy PI_TELEMETRY values.
• bun run check (typecheck + full suite): 15/15 passing.

Docs

• README: new "OpenRouter free models" section explaining the behavior and
  the telemetry gate.

Notes

• No User-Agent change was needed: pi-ai already sends Pi's UA on every
  request; OpenRouter's free-tier check is the attribution headers.
• Includes a one-line bun.lock update (peerDependency range sync).