fix(memory): make make dist produce a buildable source tarball again

make dist has failed outright since a46aea8d moved the component manuals into the repository-level docs/user-guide tree: the copy list still named src/agent-memory/docs, which no longer exists, so cp -R exited non-zero and the target aborted before writing either tarball.

$ make dist                      # at 8ba7a0f0, before this change
cp: cannot stat 'docs': No such file or directory
make: *** [Makefile:274: dist] Error 1

That failure hid a second omission: the copy list never included examples/, although Cargo.toml declares [[example]] mcp-harness with an explicit path. Cargo refuses to resolve a declared target whose file is missing, so an archive built from a merely de-docced copy list still fails every invocation that selects examples:

$ cargo build --offline --examples    # unpacked archive, no examples/
error: can't find example `mcp-harness` at path `.../examples/mcp_harness.rs`

%build runs a plain cargo build --release, which does not select examples, so rpmbuild could not report it. cargo test does build them, and the archive ships tests/ plus a Makefile whose test target runs cargo test --locked, so it is meant to be test-capable.

Neither release path in CI was affected — .github/actions/package-source copies the whole component tree and scripts/rpm-build.sh tars it minus build artefacts. make dist is the contributor-facing path (make rpm and make srpm depend on it) and is what the Makefile documents as “Create source + vendor tarballs for RPM build”.

• Drop the removed docs/ from the copy list; the spec packages CHANGELOG.md only and the manuals now live in docs/user-guide.
• Add examples/ so every target Cargo.toml declares is present.
• Guard the staged tree before tarring and name the missing path, rather than letting rpmbuild or a contributor find out later. The list is exactly what agent-memory.spec.in’s %build/%install reads plus the declared cargo target paths; a copy list that rots silently is what broke this target in the first place.
• Cargo.toml’s “Linux only” note pointed at docs/design.md, which never existed in this component; point at the repo-level docs/BUILDING.md and the remote-* targets instead.

Verified locally (crate downloads stubbed, the rest is the real recipe): before → exit 2 with cp: cannot stat 'docs'; after → exit 0 producing dist/agent-memory-0.2.7.tar.gz (367 KiB) + the vendor tarball, and the unpacked archive carries every spec-required path including examples/mcp_harness.rs. With that file removed the new guard fails the target: ERROR: examples/mcp_harness.rs is missing from dist/agent-memory-0.2.7 .... make -n dist expands cleanly, make help renders, make test-openclaw-install passes, and scripts/check-component-versions.py reports the metadata synchronized.