docs: add update and uninstall guides + README transparency section

[nathanschram]

Gap: The repo has zero uninstallation documentation and no consolidated update guide. Found during hesreallyhim/awesome-claude-code evaluation.

Added in v0.35.1:

• docs/how-to/update.md — version upgrade guide (pipx, pip, from source)
• docs/how-to/uninstall.md — removal of package, config, data, systemd unit
• README: "What Untether accesses" transparency section (network, filesystem, process, credentials)
• README: update/uninstall one-liners in Quick Start
• docs/tutorials/install.md: cross-links to update and uninstall guides
• zensical.toml, docs/how-to/index.md: nav entries

Impact: improves onboarding/offboarding experience; addresses a common reviewer ask for FOSS repos.

Closed by PR #304.