docs: init template fingerprint hint, friendlier no-device errors (#129)

[fastrevmd-lab]

Summary

Closes #129 (README portion). The code portion (init
template scaffolding, inventory error message) is covered separately by
#132 — this PR no longer touches init.rs/inventory.rs to avoid
duplicating that change.

• Documents that netconf init scaffolds desired/example/interfaces.xml
  (rename or replace example/ with your device names).
• Documents the commented host_key_fingerprint option in the generated
  inventory template, with guidance to source the fingerprint
  out-of-band rather than trusting an unauthenticated ssh-keyscan.
• Adds a "Trying it without a device" section stating plainly there is
  no bundled demo/offline mode.

Verification

cargo build -p rustnetconf-cli --locked
# clean

[fastrevmd-lab]

Addressed the changes-requested finding: the host-key fingerprint guidance in rustnetconf-cli/src/commands/init.rs and README.md no longer presents ssh-keyscan as the acquisition procedure. It now directs the reader to a trusted out-of-band source and, where the command is kept as a convenience, labels its output as unverified pending out-of-band confirmation.

Verified locally:

cargo fmt --all -- --check   # clean
cargo test -p rustnetconf-cli --locked   # 41 passed; 0 failed

CI on the PR (Test, Clippy, Rustfmt, CodeQL, cargo-audit/deny, gitleaks) was already green before this push.

[fastrevmd-lab]

Merged main into this branch to clear the BEHIND merge state (no conflicts; the diff vs main is still README.md only — init.rs/inventory.rs changes from main's #132 merge did not overlap with anything on this branch).

Verified locally: cargo fmt --all -- --check clean. No Rust files changed, so build/test/clippy results are unchanged from main. CI is re-running on the merge commit 36e0e12.

[fastrevmd-lab]

Verified the branch at the current head (36e0e12, after the earlier main-merge):

• Diff vs main is README.md only (25 insertions); the init.rs/inventory.rs changes in the diff come from main's own history via the merge, not new work on this branch.
• The last reviewer verdict (approved, non-blocking nit: the PR description's first bullet referenced an ssh-keyscan/ssh-keygen command the template no longer includes) — the description here no longer makes that claim; checked and confirmed current.
• cargo fmt --all -- --check — clean
• cargo test -p rustnetconf-cli --locked — 45 passed; 0 failed
• GitHub CI on 36e0e12: Test, Clippy, Rustfmt, YANG codegen, cargo audit, cargo-audit+cargo-deny, gitleaks, readme-version, CodeQL (actions) all pass; CodeQL (rust) was still completing at last check.

No further changes needed.

[fastrevmd-lab]

Project Structure now matches netconf init after #132.

97d4fca replaces the sentence that said init creates an empty desired/ the user fills by hand. The README now says init scaffolds desired/example/interfaces.xml as a starter config, and that you rename or replace desired/example/ so the subdirectory matches each device name uncommented in inventory.toml.

That matches rustnetconf-cli init on main (369dc0b, #132), which creates desired/example/interfaces.xml. The branch already contains that main commit. The diff vs main is README.md only.

[fastrevmd-lab]

Addressed the latest changes-requested verdict on rustnetconf#131 (MEC-2704).

That verdict asked for the README Project Structure sentence to match netconf init after #132. The sentence is already on this branch (97d4fca): init creates inventory.toml (devices commented out), desired/example/interfaces.xml as a starter config, and .netconf/state/, and you rename or replace desired/example/ so the subdirectory matches each device name uncommented in inventory.toml. That matches rustnetconf-cli/src/commands/init.rs, which creates desired/example/interfaces.xml. The earlier host-key finding stays addressed: the README still says to pin the fingerprint from a trusted out-of-band source and labels ssh-keyscan as unverified.

The branch was one commit behind main (5b0da71, #135). Merged main into docs/onboarding-init-nodevice at 651aa74. Diff vs main is still README.md only (+25). The merge brings the lockfile bump (der 0.8.0 to 0.8.2, wnaf 0.14.0 to 0.14.1).

Checks on 651aa74:

• cargo fmt --all -- --check — clean
• cargo test --workspace --all-features --locked with RUSTFLAGS=-D warnings — passed (library 560, integration_vendor_pool 9, integration_vsrx 23, rpc_reply_contract 10, rustnetconf-cli 45, yang 2, codegen 7, doc-tests 22; 0 failed)
• cargo audit — no advisories
• cargo deny check bans sources licenses — ok
• .github/scripts/check-lockfile-prereleases.py on this repo's Cargo.lock and fuzz/Cargo.lock — ok (only allowlisted ssh-key 0.7.0-rc.*)
• cargo clippy --workspace --all-targets --all-features --locked -- -D warnings — failed locally on rustc 1.99 with pre-existing clippy::double_must_use in src/transport/mod.rs (async_trait). That file is identical to main and is outside this PR's diff. GitHub Clippy on the previous head was green.

Pushed to the existing PR branch. Not merged.
#131