feat(examples): add hello-world reference app

[TeoSlayer]

Summary

The smallest end-to-end Pilot app — single IPC method (hello.echo),
sideload-safe manifest, three-command build → install → call.
Companion to #15
and TeoSlayer/pilotprotocol#240.

Why this matters: until today the wallet was the only manifest.json in
the entire ecosystem. Anyone trying to build their first Pilot app had to
reverse-engineer the supervisor contract from a production payments app.
This is the reference for what a minimal app looks like, intentionally
sideload-safe so a developer can run make install-local without going
anywhere near the catalogue / publisher-signing flow.

Layout

examples/hello-world/
  cmd/hello/main.go       single binary, one IPC method (hello.echo)
  manifest.json           sideload-safe grants (audit.log + fs $APP/)
  Makefile                bundle / install-local / uninstall targets
  README.md               build steps, supervisor lifecycle contract, trust regimes

The committed manifest.json carries binary.sha256: "REPLACE_WITH_BUILD_OUTPUT".
make bundle copies it into bundle/ and pins the real sha there, so re-builds
never rewrite the committed file. bundle/ and bin/ are gitignored.

Test plan

• make bundle && make install-local on the local daemon — installs cleanly,
  appstore list shows it with [sideloaded] badge
• pilotctl appstore call io.example.hello hello.echo '{"message":"hi"}'
  returns {"echo":"hi","sideloaded":true}
• make uninstall removes cleanly
• Reviewer: skim the README — particularly the supervisor lifecycle
  contract table and the catalogue-vs-sideload comparison. If those read
  wrong to a new developer, the rest of the example fails too.

Dependency note

make install-local uses the --local flag from web4 PR #240 + the
sideload policy from #15. Without that pair this example would have to
be publisher-signed just to install, defeating the "smallest possible
app" premise. Land those two first; this PR can wait at the back of the
chain or merge alongside.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 57 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
examples/hello-world/cmd/hello/main.go	0.00%	57 Missing ⚠️

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