Add configuration reference docs for client and session options

[Copilot]

The docs cover individual features but never enumerate the options available on CopilotClientOptions and SessionConfig, so options like system message customization, reasoning controls, BYOK providers, compaction thresholds, telemetry, and feature flags are only discoverable by reading nodejs/src/types.ts.

Adds a docs/configuration/ section with one page per option group, each with TypeScript examples verified against the current types.

Pages

• client-options.md: connection, mode, directories, logging, process-level auth, clientInfo, plus the full option table and the forUri() caveats
• system-message.md: append / replace / customize in depth — all 12 section IDs, the six override actions, transform callbacks, and the identity group + preserve pattern
• model-and-reasoning.md: model, reasoningEffort, reasoningSummary, contextTier, modelCapabilities, per-agent model overrides
• byok-providers.md: session-wide provider, bearerTokenProvider for managed identity, and the experimental named providers / models surface
• context-and-persistence.md: infiniteSessions compaction thresholds, memory, enableSessionStore, file change tracking, embedding cache storage, resume semantics
• telemetry.md, feature-flags.md, citations.md: OTel export vs. session telemetry, host-resolved flags, enableCitations

Index

• docs/README.md gains a Configuration section and a "Look up a config option" row

Section overrides require an explicit action, which is easy to get wrong from the type signature alone:

const session = await client.createSession({
  systemMessage: {
    mode: "customize",
    sections: {
      tone: { action: "replace", content: "Be terse. Bullet points only." },
      guidelines: { action: "append", content: "Cite a file path for any code." },
      code_change_rules: { action: "remove" },
    },
  },
});

Docs-only; existing feature guides are linked rather than duplicated.