[2.0] Generate docs/schema-reference.md from the config dataclasses

[ishandhanani]

Summary

Third PR of the 2.0 stack (plan: #385, Track 1 item 2). Stacked on #387; the diff against that branch is what to review.

docs/config-reference.md is hand-written prose and drifts from the schema (the power docs merged last week already reference a telemetry.provider field that #317 removed). This adds a generated, CI-checked field-level reference so keys, types, and defaults live in the code.

• src/srtctl/core/schema_docs.py walks the dataclass tree rooted at SrtConfig, the four backend types, and ClusterConfig, and renders one Markdown table per dataclass: YAML key (honoring marshmallow data_key, so gpus_per_prefill rather than _explicit_gpus_per_prefill), type (nested dataclasses linked, Literals expanded), default, and a description taken from the class docstring Attributes: block or the # comment on the field.
• srtctl schema-docs writes docs/schema-reference.md (562 lines, 38 sections). srtctl schema-docs --check exits 1 when the checked-in file is stale.
• Enforcement in three places: a CI lint step, make schema-docs-check (part of make check), and tests/test_schema_docs.py::test_checked_in_schema_reference_is_current.
• docs/config-reference.md now points at the generated file as authoritative for keys, types, and defaults; SUMMARY.md links it; CLAUDE.md tells contributors to regenerate after any schema change and where to put field descriptions so they land in the table.

Validation

• ruff, ruff format --check, and ty clean on the new module
• tests/test_schema_docs.py (9 tests: drift check, determinism, data_key handling, docstring and comment descriptions, backend and cluster sections, CLI write/check/stale)
• srtctl schema-docs --check passes on the committed file

Stack

1. [2.0] Remove the --bash direct-host execution path #386 remove --bash
2. [2.0] Replace the recipes archive with a curated examples matrix #387 examples matrix
3. this PR schema-generated reference

[ishandhanani]

Consolidated into #407, which carries these same commits as one PR against main (this description is reproduced there as one of its parts). Closing to keep review in one place; the branch stays for per-step reference via stack #398.