feat: Add SWAMP_NO_TELEMETRY environment variable to disable telemetry globally

[stack72]

Context

Issue #362 proposed three ways to globally disable telemetry. PR #364 implemented the .swamp.yaml telemetryDisabled: true config option but the environment variable mechanism was left out. This is needed for UAT/CI environments where editing .swamp.yaml on every run isn't practical.

Proposed Priority Order (highest to lowest)

1. --no-telemetry CLI flag
2. SWAMP_NO_TELEMETRY env var ← this issue
3. .swamp.yaml telemetryDisabled: true
4. Default (telemetry enabled)

Implementation Plan

1. Add isTelemetryDisabledByEnv() to src/cli/mod.ts

Add after isTelemetryDisabledByConfig(), following the same pattern as resolveModelsDir() which already uses Deno.env.get("SWAMP_MODELS_DIR"):

/**
 * Checks whether telemetry is disabled via SWAMP_NO_TELEMETRY environment variable.
 * Any value other than "0", "false", or empty string disables telemetry.
 *
 * @internal Exported for testing
 */
export function isTelemetryDisabledByEnv(): boolean {
  const val = Deno.env.get("SWAMP_NO_TELEMETRY");
  if (val === undefined) return false;
  return val !== "0" && val !== "false" && val !== "";
}

2. Update runCli() in src/cli/mod.ts

// Before:
const telemetryDisabled = isTelemetryDisabled(args);

// After:
const telemetryDisabled = isTelemetryDisabled(args) || isTelemetryDisabledByEnv();

This gives the correct precedence: the flag and env var are both pre-parse checks evaluated before initTelemetryService() (which checks the config file internally).

3. Add tests to src/cli/mod_test.ts

Following the exact pattern of the resolveModelsDir env var tests (save original, modify, restore in finally). Cover:

• env var not set → telemetry enabled
• env var set to "1" → disabled
• env var set to "true" → disabled
• env var set to "0" → enabled (explicit opt-in override)
• env var set to "false" → enabled (explicit opt-in override)
• env var set to "" → enabled (empty = unset)

4. Update README.md

Extend the "Disabling Telemetry" section to include:

Via environment variable (useful for CI/UAT environments):

```bash
export SWAMP_NO_TELEMETRY=1
swamp workflow run my-workflow

Priority order (highest to lowest): --no-telemetry flag → SWAMP_NO_TELEMETRY env var → .swamp.yaml telemetryDisabled: true.

## Files to Modify

- `src/cli/mod.ts` — Add `isTelemetryDisabledByEnv()`, update `runCli()`
- `src/cli/mod_test.ts` — Add 6 tests for the new function
- `README.md` — Document env var and precedence order

Closes #362 (remaining piece)