CLI/docs: steer quickchr users to centrs for validated RouterOS operations

[mobileskyfi]

Context

Decision from #39: quickchr exec stays a raw/simple channel. Users who want validation, structured envelopes, provenance, fan-out, and richer RouterOS diagnostics should move to centrs after quickchr creates/provisions a VM.

quickchr should make that path discoverable without depending on centrs or making normal exec noisy.

Strawman

Add lightweight discovery in the places where users naturally finish creating or inspecting a machine:

• quickchr help exec: note that raw quickchr exec is intentionally unvalidated, and point to centrs execute --quickchr <name> -- <command> for validate/explain/run once centrs supports --quickchr.
• quickchr inspect / quickchr env docs: name these as the handoff surfaces for external tools, including centrs.
• quickchr setup / wizard finish flow: consider detecting centrs on PATH and showing the next command. If centrs is missing, consider an optional install prompt or a plain install tip (bun install -g @tikoci/centrs). Keep this TTY-only and never part of non-interactive start/exec.
• README/MANUAL: document the boundary in one short place: quickchr owns CHR lifecycle and QEMU-local channels; centrs owns validated RouterOS operations.

Guardrails

• No hard dependency on centrs.
• No repeated warning on every quickchr exec; users who choose raw exec should not be nagged.
• No automatic install outside the interactive wizard/setup path.
• Keep the wording short; this is a pointer, not a second centrs manual.

Done-when

A quickchr user can discover the centrs path from help/setup/docs, the wizard install/tip decision is recorded, and the common path remains quiet and dependency-free.

[mobileskyfi]

Docs/help nuance from the centrs review:

Please keep the pointer focused on validated RouterOS operations after VM setup, for example centrs retrieve --quickchr <name> /system/resource and centrs execute --quickchr <name> -- <command>. Do not imply that centrs btest --quickchr is part of the handoff; that would require quickchr to provision or understand feature-specific services/custom forwards, which is outside quickchr's core setup boundary.

For an install hint, bun add -g @tikoci/centrs is the clearer Bun spelling for a global package install. Keep it TTY/setup/docs-only as this issue already says, not a recurring exec warning.