docs: add a code of conduct

[Considus]

What this changes

Adds CODE_OF_CONDUCT.md. This was the last gap on GitHub's community checklist, which the repo was sitting at 6 of 7 on.

Deliberately not the Contributor Covenant. Its enforcement ladder commits to a review and investigation process that a single maintainer doesn't have, and an unmet published promise is worse than no promise if it's ever tested. This version commits only to what would actually happen, which is that I read everything, delete what crosses the line and say why, and block if it continues. It states there's no committee and no appeals process rather than implying there is one.

Contact is security@considus.com rather than a new conduct@ mailbox, so it lands somewhere already watched, and it points actual security reports at SECURITY.md so they don't end up in a public issue.

The real value is moderation cover rather than the score. If someone is abusive in an issue, deleting the comment becomes a stated rule being applied rather than a judgement call.

How it was tested

Documentation only, no app or test changes. GitHub picks CODE_OF_CONDUCT.md up from the repository root automatically and links it in the sidebar.

• swift run coreverify (52/52 green)
• swift test
• xcodebuild test on a simulator
• Checked on a physical device

CI runs the full set regardless, and all four required checks have to pass before this can merge.

Non-negotiables

• No analytics, no telemetry, no off-device data transmission
• Zero-knowledge and encryption-always-on still hold
• kSecAttrSynchronizable: false on every Keychain item
• No changes to the frozen crypto contract bytes in Sources/CatchlightCore/Crypto/
• No new third-party dependencies

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