Add first-class `X-Org-Id` support on the SDK `Client` builder

[chaliy]

Summary

Add first-class support for the X-Org-Id header on everruns-sdk's
Client. Today, API-key principals with access to multiple
organizations hit:

Multiple organizations available. Specify the target organization via
the X-Org-Id header.

…and there is no typed way to set it — you have to monkey-patch the
registry source or rely on an environment variable that isn't part of
the public API.

Context

Found while running an Everruns agent
(everruns/a11y-nanny) against
dev.everruns.com with an API key that had access to more than one org.

The workaround I used in-tree was to patch
everruns-sdk-0.1.8/src/client.rs::headers() to read EVERRUNS_ORG_ID
from the environment and inject it as X-Org-Id:

if let Ok(org) = std::env::var("EVERRUNS_ORG_ID") {
    if let Ok(v) = HeaderValue::from_str(&org) {
        headers.insert("X-Org-Id", v);
    }
}

That works but:

• It's not discoverable from the type signature.
• Env-var-only means callers in containers / hosted runtimes have to
  remember to set it.
• It silently drops invalid header values.

Proposal

Add a typed builder option on Client::builder() (or whichever builder
the SDK exposes) plus a documented precedence:

1. Explicit builder.org_id(...) / Everruns::builder().org_id(...).
2. EVERRUNS_ORG_ID env var.
3. Omitted (single-org accounts don't need it).

Behaviour:

• When set, every request carries X-Org-Id: <value>.
• Invalid header value returns a typed error at build time, not
  silently swallowed.
• from_env() picks up EVERRUNS_ORG_ID automatically, same as
  EVERRUNS_API_KEY.

Suggested API

let client = Everruns::builder()
    .api_key(std::env::var("EVERRUNS_API_KEY")?)
    .org_id("org_019...") // new
    .build()?;

// or purely from env
let client = Everruns::from_env()?; // also honours EVERRUNS_ORG_ID

Acceptance criteria

• Client::builder().org_id(...) compiles and sends
  X-Org-Id on every request.
• Everruns::from_env() reads EVERRUNS_ORG_ID if present.
• Precedence: builder value wins over env var.
• Invalid header value returns a typed build-time error.
• README Quick Start updated with a multi-org example.
• Integration test hits a mock server and asserts the header is
  forwarded.

Related

• CLI-side counterpart (already shipped in the everruns/everruns
  monorepo): commit ee17a05 on branch
  claude/setup-a11y-audit-agent-1Tzkq adds the same env-var
  forwarding to /v1/agents/import. Tracked in Linear as EVE-338.
• Session that exposed the gap:
  a11y-audit-output/session-019da1bd6745737d92a1266e84475972.jsonl
  on the branch above.