Add TTY check and debounce behavior; update README and tests

[Astro-Han]

Motivation

• Make the bell plugin quieter and safer by only ringing in interactive terminals and preventing rapid duplicate rings.
• Document the new runtime behavior and publish a patch version bump to 0.1.1.
• Add unit tests to validate TTY checking, debounce timing, and per-session keys.

Description

• Bumped README install example to opencode-bell@0.1.1 and added a "Behavior details" section documenting that the plugin only rings when process.stdout.isTTY is true, debounces repeated rings within 1200ms, and uses session-specific keys when event.properties.sessionID is present.
• Added test helpers withStdoutStub and withMockedNow and expanded test/plugin.test.js with tests that assert ringing for supported events, silence for unsupported events, TTY gating, debounce behavior across timestamps, and session-specific debounce keys.
• Ensured the plugin still tolerates missing event.properties by keeping the existing test that verifies no rejection when properties is absent.

Testing

• Ran the unit test suite (node:test via the repository npm test command) which executed the updated test/plugin.test.js tests.
• The new tests for TTY gating, event support, debounce timing, and session-specific keys passed successfully.
• The existing regression test that ensures no throw on missing event.properties also passed.

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Codex Task