Keep self-hosted uninstall inside its installation

[SaladDay]

What

• install defaults to <Runtime home>/environments/<environment-id> for every mode (one definition, defaultDirectory), says so in the flag help, and refuses a non-empty --install-dir that holds no installation, naming it. A directory holding only installer entries (a failed earlier install) is accepted so reruns stay clean.
• uninstall deletes only the installation's entries (one shared list, nativeInstallationEntries, used by install and uninstall), removes the directory only when it is then empty, and prints Kept: <path> for everything else, including a credential file or capability directory outside the installation. It deletes bin/ and daemon/installation.json last so a failed run can be rerun, names the path it could not delete, and no longer touches native-download. The running executable is matched with os.SameFile. The output says the Session's install command cannot be reused and what reconnects it.
• bootstrap.sh treats 22:416 on a resumed download as complete and lets the checksum decide.
• Both bootstraps (and the POSIX command's own fetch) keep the grant out of every child except the oac-daemon install call.
• v1.NativeInstallGrantEnv in contracts/agents-api/v1 is the single definition of the grant variable name; the Core and daemon copies are gone.
• self-hosted.md and the executor-credentials contract (EN+zh) are corrected: uninstall scope, PowerShell uninstall form, PowerShell history behaviour, grant handling, directory rule.

Why

Review findings P0-1, P1-1, P2-1 to P2-8 (review-self-hosted.md). P2-5 preflight: POST /installation cannot report a claimed credential without a new field, and the original installation's own rerun (same secret) must pass it, so it is left out.

Verification

• go test ./apps/daemon/internal/cli -run 'Native|Uninstall|Install|RemoveTree|Interrupt|Onboard|Discovery|Start': pass
• go test ./services/core/internal/nativeinstaller ./contracts/...: pass
• go vet ./... and GOOS=windows go vet ./apps/daemon/internal/cli: pass
• scripts/check-names.py: pass; website/tests/translations.test.mjs: tests 1-2 pass
• New tests: foreign files survive uninstall and are reported; foreign directory refused; default directory; failed uninstall rerunnable; complete .part installs; grant reaches only the installer.

Skipped

• Translations test 3 (needs vitepress, website deps not installed)
• PowerShell paths (bootstrap.ps1, Windows deleteAfterExit): cannot run here; CI's Windows job covers them

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