feat(build): add --prune flag to build.{sh,ps1} [skip-tdd: wrapper]

Symmetric to install-side --prune (e871c20). Removes
dist/<name>.skill files whose <name> is not in skills/*.

Design choices match install:
- Combined flag (build + prune in one run)
- Global scan, ignores -Names / positional filter
- Default off, print-and-delete, no confirmation

Bash wrinkle: when build.sh delegates to powershell.exe -File
build.ps1 (Windows-without-zip case), --prune is NOT forwarded.
Bash runs prune itself at the end of the script against the
shared dist/. Keeps the delegation surface narrow and the prune
logic single-sourced per shell.

[skip-tdd: wrapper] — same carve-out as install-side, smoke-test
evidence: fake dist/fake-stale-{sh,ps}.skill files created in real
dist/, ran build.{sh,ps1} --prune, verified fakes removed, real
caveman.skill etc. left intact.

Wiki .wiki/concepts/install-cross-platform.md extended:
- title broadened (Install → Install / Build)
- new "Build-side --prune" section
- scope note: dist-hermes/ is separate (managed by build-hermes.py)

Closes [install-ps1-build-prune-followup].

.wiki/log.md

@@ -63,3 +63,5 @@ Parseable: `grep "^## \[" .wiki/log.md | tail -20`.
 ## [2026-05-25] decision | session-handoff-skill-design — design rationale for the `session-handoff` skill captured in wiki after cluster 7/7 closure; sliding overwrite of `.tasks/NEXT_SESSION.md`, phrase whitelist + substantive-commit heuristic, opt-in PostToolUse hook, orient+ask default, source: `~/projects/.workshop/.archive/2026-05-24-session-handoff-skill.md` Round 1 + Round 2
 
 ## [2026-05-25] decision | install-cross-platform — `install.{ps1,sh}` paired-script parity contract documented; `--prune` / `-Prune` flag rationale (combined-with-install, global-scan ignores names filter, default-off, print-and-delete no prompt); shipped in commit `6cf0e98` with `[skip-tdd: wrapper]` carve-out + smoke-test evidence; closes 2/3 of `[install-ps1]` acceptance (the doc + flag), `dist/`-prune analogue deferred to `build` scripts
+
+## [2026-05-25] decision | install-cross-platform extended to build scripts — `build.{ps1,sh}` get the symmetric `--prune` / `-Prune` flag (removes `dist/<name>.skill` where `<name>` is not in `skills/`). Bash delegation to `powershell.exe -File build.ps1` does NOT forward the flag — bash runs prune itself against the shared `dist/`. Both paths smoke-tested with fake stale .skill files against real dist/. Closes `[install-ps1-build-prune-followup]`.