docs: OSS adoption upgrade

[solomonneas]

Brings agent-notify up to the Escoffier OSS adoption standard. Docs and repo metadata only; no source, build, or manifest changes.

Changes

README

• Opens with what / why / how-it-differs in the first three sentences, with the differentiator (no telemetry, no vendor push service) on the first screen.
• Centered title, bold one-liner, centered shields.io badge row (CI, release, Go version, MIT license), and a prominent Website link to https://agent-notify.escoffierlabs.dev (verified HTTP 200).
• New keyword-rich ## What it does section with the primary keyword in the first 100 words.
• New ## Verify the wiring section with real, verified doctor and status output captured from the built binary in an isolated environment (no fabricated output).
• New ## Why not <alternatives>? and ## What agent-notify is not sections.
• Preserves all accurate existing content (install, quickstart, routing, hook integrations, exit codes, channel formatting, limitations).

Maintainer-health files (new)

• SECURITY.md - private report path, trust model, in/out-of-scope.
• CONTRIBUTING.md - contribution scope, channel/hook adapter guides, no-telemetry invariant.
• CODE_OF_CONDUCT.md - Contributor Covenant 2.1.
• .github/ISSUE_TEMPLATE/{bug.yml,feature.yml,config.yml} - blank_issues_enabled: false with website + security contact links.
• .github/PULL_REQUEST_TEMPLATE.md - includes a no-PII / content-guard checkbox.
• CHANGELOG.md - Keep a Changelog format with an Unreleased entry plus the released versions.

Repo metadata (via gh, additive)

• Description and homepage confirmed; added topics codex, privacy, webhook.

Checklist

• Website link re-verified (HTTP 200)
• content-guard scan clean on every changed/new file; pre-push hook passed
• Real-output blocks are verified, not fabricated
• Example IPs use 192.0.2.x; no home paths, hostnames, or real private IPs
• Conventional commit, no AI co-authorship trailer
• No source / build / manifest changes
• Do not merge or cut a release in this PR

Please review with Codex only (no CodeRabbit / no bots).