feat(instrument): Smolagents adapter closure — sample + doc + alias (fixes mature label honesty)

[mmercuri]

Why this PR exists

scripts/emit_adapter_manifest.py lists smolagents in the _MATURE set
(line 156), alongside openai, anthropic, aws_bedrock, etc. The
manifest schema docstring defines mature as:

has dedicated unit-test file in tests/instrument/ AND a reference
doc in docs/adapters/.

Audit finding: smolagents had the test file
(tests/instrument/adapters/frameworks/test_smolagents_adapter.py) but
no docs/adapters/frameworks-smolagents.md and no
samples/instrument/smolagents/. The atlas-app catalog UI was
therefore advertising a tier the SDK could not back, in violation of
CLAUDE.md's "no fake data, no inflated labels, complete means
complete" rule.

This PR closes the artifact gap so the existing mature label is
truthful — no scope expansion, no decomposition of the 388-LOC
lifecycle.py (that is a separate follow-up PR).

What's in the PR

1. Reference doc (docs/adapters/frameworks-smolagents.md)

Mirrors the structure of frameworks-agno.md:

• Install + Python/Pydantic version compatibility
• Quick-start with real CodeAgent + HfApiModel
• Events emitted (8 events across L1/L2/L3/L4a/L5a + cross-cutting)
• SmolAgents specifics: manager → managed agent recursion,
  CodeAgent vs ToolCallingAgent, idempotent re-instrumentation,
  per-thread duration_ns correctness
• Capability matrix: what's auto-wrapped vs caller-driven
• Capture config recipes including capture_content=False
  (PII-sensitive)
• BYOK note + backward-compat section pointing at the
  STRATIXSmolAgentsAdapter deprecation alias

2. Offline-runnable sample (samples/instrument/smolagents/)

• main.py, __init__.py, README.md
• Diverges from the samples/instrument/agno/ "requires OPENAI_API_KEY"
  pattern intentionally — the sample uses a duck-typed _FakeAgent so
  every developer can run python -m samples.instrument.smolagents.main
  with zero installs, zero network, zero API keys. The README
  documents the live-path swap (real CodeAgent + HttpEventSink).
• Output is deterministic — 3 events emitted in known order, suitable
  for CI snapshotting.

3. STRATIX → LayerLens deprecation alias

• STRATIXSmolAgentsAdapter re-exposed via PEP 562 module
  __getattr__ in src/.../smolagents/__init__.py, with
  DeprecationWarning on first access.
• Listed in __all__ and __dir__ for IDE / dir() discoverability.
• New tests: test_legacy_stratix_alias_warns_and_resolves,
  test_unknown_attribute_still_raises_attribute_error.
• The lifecycle docstrings were already LayerLens-branded; no STRATIX
  strings to sweep within the smolagents adapter package.

Out of scope (explicitly)

• Decomposing the 388-LOC lifecycle.py into sub-modules — separate
  follow-up PR.
• Auto-wrapping smolagents tool / model layers — needs upstream
  callback hooks; documented as caller-driven for now.

Acceptance — STRICT (all green)

$ uv run pytest tests/instrument/adapters/frameworks/test_smolagents_adapter.py -x
14 passed in 1.10s

$ uv run mypy --strict src/layerlens/instrument/adapters/frameworks/smolagents
Success: no issues found in 2 source files

$ uv run ruff check src/layerlens/instrument/adapters/frameworks/smolagents \
                   tests/instrument/adapters/frameworks/test_smolagents_adapter.py
All checks passed!

$ uv run python -m samples.instrument.smolagents.main
Agent output: echo: What is 2 + 2?
Emitted 3 event(s):
  -     environment.config  agent=demo-agent
  -            agent.input  agent=demo-agent
  -           agent.output  agent=demo-agent

Lazy-import + default-install guards still pass for smolagents
(tests/instrument/test_lazy_imports.py — both
test_layerlens_import_does_not_pull_frameworks and
test_instrument_import_does_not_pull_frameworks green).

CLAUDE.md commercial-grade compliance

• No TODOs, no skeleton, no placeholders.
• Sample runs offline end-to-end with no real LLM / network calls.
• All renames preserve backward compat via the deprecation alias.
• No co-author trailer. Draft PR.