Resolve local paths against the suite file and widen --check

[bgrewell]

Five things the documentation had to apologize for. Rather than document the wart, fixed the behaviour.

One convention for local paths

Absolute paths are used as-is, ~ expands to the invoking user's home, and everything else resolves against the directory holding the suite file — not the process working directory.

That now covers file-step source/dest, docker volumes, LXD disk source, SSH key and known_hosts, LXD client/server certificates, and compose_file, joining docker.images[].dockerfile and !!load_from which already worked this way.

A suite is portable as a result. Verified by running the same suite from /:

[ local ] push a fixture named relative to the suite file ... done
00001: [ local ] the fixture arrived ... passed

Before this, that same run failed with open fixtures/app.conf: no such file or directory.

--check now validates what it was assumed to

• Required fields — host on ssh, image on docker, compose_file on docker-compose. An ssh node missing host used to pass the check and then fail the run dialling :22.
• Unknown option names, reported with the accepted set:

  Error: node "web": unknown option "privilaged" for a docker node (accepted:
  capabilities, command, container_name, entrypoint, env, exec_opts, image,
  networks, ports, privileged, volumes)
  

  Options decode through a JSON round-trip that discards unrecognised keys, so privilaged left privileged at its default while the suite read as though it were set — a silent false green.
• Cross-node constraints — duplicate node names and more than one local node. Both lived inline in the node factory, so --check (which substitutes mock nodes) never reached them. Extracted to ValidateNodeSet, called from both.

Two missing options

• command / entrypoint on docker nodes override the image's CMD/ENTRYPOINT. A bare distribution image can now host a node — command: ["sleep", "infinity"] — instead of exiting the moment it starts.
• container_name (docker) and instance_name (lxd) decouple the platform identifier from node identity. Both default to the node name, so nothing changes unless set. The node name remains what node: references, what reports show, and what the container's hostname is set to.

Note on strictness

Unknown option names are now a hard error, not a warning. That is the stricter reading — it is consistent with how the project has treated other silent-no-op classes, and a stray key in an existing suite will now fail rather than be ignored. Easy to soften to a warning if you would rather not break suites carrying dead keys.

Verification

New tests cover path resolution (including ~, ../, absolute, and the in-memory fallback), suite-dir stamping through load, name defaulting and override for both platforms, command/entrypoint decoding, unknown-option rejection across node types, and ValidateNodeSet. go build, go vet, gofmt, and the full suite are green, and the docs site builds.

Docs updated throughout — the notes describing the old behaviour are gone rather than reworded.