Restructure the README to the portfolio standard

[adamXbot]

Brings the README into the portfolio-wide layout: hero, badge row, disclosure block, rule, overview, ## What it does, ## Get it, ## Contributing, ## Licence. The existing voice and the accurate prose are kept — this is a restructure plus a correctness pass, not a rewrite.

Wrong facts removed

• The Install section pointed at a download that does not exist. It said to download the latest ClaudeWatch.zip from the Releases page, unzip, and drag to /Applications. gh release list is empty and there are no tags, so there has never been anything on that page. Replaced with build-from-source instructions and a plain statement that there is no packaged download and no Homebrew formula yet.
• The release badge could not resolve. https://img.shields.io/github/v/release/adamXbot/ClaudeWatch returns release: no releases or repo not found. Dropped until a non-prerelease release exists.
• "makes no network connections" — not true. Sources/ClaudeWatch/Updater.swift constructs SPUStandardUpdaterController(startingUpdater: true, …), so Sparkle begins its scheduled-update lifecycle at launch against the SUFeedURL that build.sh writes into Info.plist.
• "No telemetry, no network, no data leaves your machine" — also not true. NotificationEngine holds a URLSession and posts to user-configured Discord, Slack, Teams and generic-JSON webhooks. The overview now says what is actually true: it never writes to your transcripts and there is no telemetry, and it names both network paths.
• The signed/notarised release note described a path that has never run. The release job in build.yml only fires on a v* tag, and no tag exists. Only the unsigned first-launch caveat is kept.
• "Two icons appear in your menu bar" was unconditional. Per SourceAvailability and isVisible(_:), one menu-bar item appears per source that actually has transcripts, with a manual show/hide override in Settings.

No dead relative paths were found in the old README; every path and URL in the new README and CONTRIBUTING.md was checked (test -e for paths, curl for URLs) and returns < 400, with the one exception noted at the bottom.

Real content that was missing

Added from the source, not invented: the Codex event kinds (exec_command, write_stdin, apply_patch), the active-sessions strip and the bell menu-bar icon when a session is waiting, and the notification rules with Keychain-stored webhook URLs. None of these were documented.

Content moved, not deleted

New CONTRIBUTING.md takes the project layout, the --dump and --render-test headless modes, the justfile recipes, what CI genuinely runs, and how the tag-triggered release job degrades when signing secrets are absent.

Badges

Status, CI, licence. The CI badge resolves to passing — build.yml runs on pushes to main and there are successful runs there. The licence badge resolves to MIT — LICENSE exists.

Size

README 4624 -> 5060 bytes, under the ~6 KB target. Plus 3378 bytes of moved content in CONTRIBUTING.md.

Known, expected

The status badge renders "resource not found" and its STATUS.md link 404s until the status hub PR merges in adamXbot/.github. The endpoint path is correct and will resolve then.

Separately, and out of scope for this PR: build.sh and build.yml both point maintainers at a RELEASING.md that does not exist in the repo.