docs: address review on context7 rules — op-mode set/add, edit-subtree

Three corrections from Copilot review on #1946:

Rule 2: Mode framing was technically wrong. `set ... ` is not
unique to config mode — op mode has its own `set`/`add` commands
for image management (`add system image`, `set system image
default-boot`). Replace the `set/delete/show` listing with mode
entry mechanics (`configure` enters config; op mode is the default
after login) and acknowledge image-mgmt commands live in op mode.

Rule 8: `add system image` and `set system image default-boot` are
operational-mode commands (verified: docs/installation/update.md
wraps `add system image` in `{opcmd}`; docs/installation/image.md
does the same for `set system image default-boot`). The upgrade
also is not in effect until reboot. Update to: op mode + reboot
to activate.

Rule 10: "Configuration is hierarchical — paths extend without
re-stating it" was misleading. At the top level (just after
`configure`), every `set`/`delete` line must include the full
path. Relative paths only work inside an `edit <subtree>` context.
The original phrasing would have led an LLM to omit parent nodes
and emit invalid CLI examples.

All revised rules verified ≤255 chars.

🤖 Generated by [robots](https://vyos.io)

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