Extract Scheduler init methods and add skills to enforce the splitting requirements (#26271)

.claude/rules/modify-component-must-read.md

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+# Must-Read Skills Before Modifying Components
+
+Before modifying the following components, read the listed skill first.
+
+- **Speculative decoding code** (anything under `python/sglang/srt/speculative/`, related attention backends, scheduler accumulators, IPC fields, observability metrics, or CLI flags) → [`speculative-naming`](../skills/speculative-naming/SKILL.md)
+- **`Scheduler` / `TokenizerManager` / `ModelRunner` `__init__`** (`python/sglang/srt/managers/scheduler.py`, `python/sglang/srt/managers/tokenizer_manager.py`, `python/sglang/srt/model_executor/model_runner.py`) → [`large-class-init-style`](../skills/large-class-init-style/SKILL.md)

.claude/rules/speculative-naming.md

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-# Speculative Decoding — Naming Conventions
-
-When adding, renaming, or reviewing identifiers in speculative decoding code (anything under `python/sglang/srt/speculative/`, related attention backends, scheduler accumulators, IPC fields, observability metrics, or CLI flags), read the [`speculative-naming`](../skills/speculative-naming/SKILL.md) skill first.

.claude/skills/large-class-init-style/SKILL.md

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+---
+name: large-class-init-style
+description: '`__init__` style for SGLang `Scheduler`, `TokenizerManager`, and `ModelRunner`. Use when modifying the `__init__` of any of these three classes, or reviewing changes that add new construction logic to them.'
+---
+
+# `__init__` Style for Scheduler / TokenizerManager / ModelRunner
+
+Apply when modifying the `__init__` of:
+
+- `Scheduler` — `python/sglang/srt/managers/scheduler.py`
+- `TokenizerManager` — `python/sglang/srt/managers/tokenizer_manager.py`
+- `ModelRunner` — `python/sglang/srt/model_executor/model_runner.py`
+
+## Why
+
+- Downstream forks override one piece (tokenizer, KV cache, IPC, …).
+- Inline logic forces them to copy the whole `__init__`, which rots against upstream.
+- Splitting into `init_*` helpers lets them override exactly what they need.
+- Reference shape: `TokenizerManager.__init__` in `python/sglang/srt/managers/tokenizer_manager.py`.
+
+## Rules
+
+- **`__init__` is an orchestrator.** Sequence of `self.init_*(...)` calls + minimal glue. No non-trivial construction inlined.
+- **One helper per overridable unit.** Each `init_*` = one concern a subclass might swap. Don't lump.
+- **Naming:** `init_<thing>` (snake_case, names the component). Conditional construction → `maybe_init_<thing>`, gate inside the helper.
+- **No silent state coupling.** A helper only reads `self.*` set by earlier helpers. Ordering lives in `__init__`. Shared intermediates → pass as args, not via `self.*`.
+- **New logic = new helper.** Default to adding `init_<thing>`, not another inline block. One-line `self.foo = server_args.foo` is fine; structured logic is not.
+- **Preserve override points.** Prefer additive changes to existing `init_*` signatures. Breaking changes → call out in PR.
+
+## Scope
+
+Only the three classes listed above. Not other manager-style classes, not small dataclass/utility constructors.