Add structured logging, MCP request logging, and an OpenTelemetry pipeline across server apps

[FelineStateMachine]

Motivation

Outside of apps/cloud, the server apps have no configured logger (Effect's
default pretty printer), no log or metric export, and no tracer, so the
roughly 145 existing Effect.withSpan call sites in core packages export
nothing. MCP protocol traffic is only visible through the EXECUTOR_MCP_DEBUG
console hook. This makes self-hosted and local deployments hard to operate and
debug.

This PR adds structured logging, MCP request logging, and an OTLP export
pipeline (traces, logs, metrics) to all four server apps. When no OTLP
endpoint is configured, export is disabled and only the structured console
logger is active. Runtime behavior is otherwise unchanged.

Changes

Two commits.

1. Structured logging, MCP request logging, and OTLP telemetry

• New package packages/core/observability (@executor-js/observability).
  It exports observabilityLayer({ serviceName, endpoint, headers, logLevel }),
  which provides a structured JSON console logger and OTLP/HTTP exporters for
  traces, logs, and metrics. It is built on effect/unstable/observability
  (fetch-based, no async_hooks), so the same layer runs on Bun and on
  Cloudflare workerd. No new npm dependencies.
• Structured mcp.* log events in packages/hosts/mcp and
  packages/hosts/cloudflare: mcp.tool.start and mcp.tool.end with
  outcome (success, paused, approval_required, error) and duration,
  mcp.tool.internal_error with a correlation id, session lifecycle events
  (created, closed, idle_expire, init_failed, alarm), mcp.auth.outcome,
  dispatch results, and elicitation events at debug level. Two metrics:
  mcp.tool.calls (counter) and mcp.tool.duration_ms (histogram), tagged by
  tool and outcome. Code payloads are not logged; only
  mcp.execute.code_length, matching the existing span attribute convention.
• Wiring in apps/local, apps/host-selfhost, apps/host-cloudflare, and
  apps/cloud. Cloud keeps its existing trace pipeline and error-capture
  correlation and gains only the logs and metrics layers (the shared package
  takes traces: false for this). apps/local additionally runs its
  in-process MCP surface through a dedicated runtime so tool calls log through
  the same pipeline and flush on shutdown.
• Docs: package role in AGENTS.md, configuration section in RUNNING.md.
• EXECUTOR_MCP_DEBUG is unchanged and coexists with the new events.

2. SpanKind.CONSUMER on MCP tool-call spans

mcp.host.tool.execute and mcp.host.tool.resume spans now carry OTel
SpanKind.CONSUMER. Without a kind, these are root INTERNAL spans with no
HTTP attributes wherever no HTTP server span wraps the call (for example the
stdio transport), and span classifiers in observability backends commonly
discard such roots. CONSUMER marks them as remotely initiated units of work
processed by this server, which matches their semantics.

Design notes

• Each app builds the layer from its own environment source (process env on
  Bun, bindings on Workers) and merges it into the boot seam of
  ExecutorApp.make. Shared packages contain no environment reads and no
  conditional wiring.
• The console logger writes JSON lines to stderr, not stdout, because the MCP
  stdio transport uses the process's stdout as its JSON-RPC channel. A stdout
  logger corrupts the protocol stream; the existing stdio integration test
  catches this.
• Log lines include trace_id and span_id from the fiber's current span, so
  console output correlates with exported traces the same way OTLP log records
  do.
• The MCP tool server already captures its Effect context at construction and
  re-enters it per SDK callback, so the host app's logger and tracer reach
  every tool call without new plumbing. SDK lifecycle callbacks that run
  outside Effect (for example transport close) emit single-line JSON directly.
• The OTLP logger is added to the current logger set rather than replacing it,
  so console and OTLP both receive every record.

Configuration

Variable	Default	Effect
OTEL_EXPORTER_OTLP_ENDPOINT	unset	OTLP/HTTP base URL; /v1/{traces,logs,metrics} appended. Unset disables export.
OTEL_EXPORTER_OTLP_HEADERS	none	Standard key=value,key2=value2 format.
LOG_LEVEL	info	Minimum level for console and OTLP logs.

On the Workers apps these are bindings rather than process env.

Testing and verification

• format:check, lint, typecheck, and test pass.
• New unit tests: logger output shape and level filtering, header parsing,
  disabled-when-unconfigured behavior, OTLP layer teardown; MCP host tests
  assert mcp.tool.start/mcp.tool.end fire with outcome, duration, and
  execution id, and that defects produce the correlation-id error log.
• Payload check against a local OTLP collector stub: spans arrive on
  /v1/traces, trace-correlated log records on /v1/logs, both metric series
  on /v1/metrics.
• Deployed to a self-hosted OTLP backend from this branch (apps/host-selfhost
  and apps/local). Screenshots below. With the endpoint unset, there is no
  network activity and existing e2e tests (including the cloud
  error-correlation contract test) pass unchanged.

MCP tool call classified as a task via SpanKind.CONSUMER, with tool
attributes and child spans:

Endpoint grouped by parametrized route with database child spans:

Disclosure

This change was developed with an LLM coding assistant (Claude), directed and
reviewed by the submitter. Design decisions, code, tests, and the verification
above were produced in that workflow and checked against the repository's full
test gates before submission.