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/concepts/install-cross-platform.md

@@ -1,12 +1,12 @@
 ---
-title: Install Cross-Platform Parity (PS + Bash)
+title: Install / Build Cross-Platform Parity (PS + Bash)
 type: concept
 updated: 2026-05-25
 ---
 
-# Install Cross-Platform Parity (PS + Bash)
+# Install / Build Cross-Platform Parity (PS + Bash)
 
-Sibling concept to `install-portability.md` (POSIX-shell compat). This one is about the **paired-script parity contract** between `scripts/install.ps1` (PowerShell) and `scripts/install.sh` (bash).
+Sibling concept to `install-portability.md` (POSIX-shell compat). This one is about the **paired-script parity contract** between `scripts/install.ps1` / `scripts/install.sh` (install) and `scripts/build.ps1` / `scripts/build.sh` (build). Both pairs share the same conventions and the same `--prune` / `-Prune` flag pattern.
 
 ## Why two scripts
 
@@ -43,14 +43,35 @@ Added 2026-05-25 (commit `6cf0e98`). Motivating case: after retiring `using-syno
 
 - **Default off.** Must be passed explicitly. Idempotent re-installs (the common case) don't suddenly delete anything.
 
-## What `--prune` does NOT do
+## What install-side `--prune` does NOT do
 
-- It does not remove `dist/<name>.skill` artefacts. That's a build concern, not install. The matching `--prune` flag on `build.sh` / `build.ps1` is a separate task (`[install-ps1]` acceptance was scoped to install scripts; `dist/` is a build artefact directory).
 - It does not touch plugin-installed skills under `~/.claude/plugins/<plugin>/skills/<name>/`. Those are managed by the plugin system, not this repo.
 - It does not warn if the about-to-be-deleted dir contains user-edited content. The contract is that `~/.claude/skills/<name>/` is a managed copy of `skills/<name>/` — anything else is user-error.
+- It does not remove `dist/<name>.skill` build artefacts. That's the build-script's `--prune` (see next section).
+
+## Build-side `--prune` / `-Prune`
+
+Added 2026-05-25 — natural extension of the install-side flag to the build pair (`scripts/build.sh` and `scripts/build.ps1`). Same design choices, applied to files instead of directories: after the build loop, walk `dist/*.skill` and remove any whose `<name>` (basename minus `.skill`) is not in `skills/*`.
+
+Identical to install-side: combined flag, global scan ignores the name filter, prints `pruning: <name> -> <path>` per removal, default off, no confirmation prompt.
+
+The bash path has one extra wrinkle: when `build.sh` is run on Windows without `zip` and delegates to `build.ps1` via `powershell.exe -File`, the `--prune` flag is **not** forwarded to the delegated PS process. Bash runs the prune step itself at the end of the script, against the same `dist/` directory. This keeps the delegation surface narrow (no flag-translation bugs) and the prune logic single-sourced per shell.
+
+`build.ps1` invoked directly (without the bash wrapper) handles `-Prune` natively.
+
+## What build-side `-Prune` does NOT do
+
+- It does not unblock build for skills that have been renamed mid-flight. A user who renamed `skills/foo/` → `skills/bar/` should still run a fresh build (`build.sh bar`) — prune only catches stale archives whose source dir is gone, not stale archives whose source was renamed (those become orphans of a different source, indistinguishable from intentional foreign artefacts).
+- It does not touch `dist-hermes/` — that directory is managed by `scripts/build-hermes.py` and follows its own rules (whole-directory rebuild per skill). Hermes has no current prune mechanism; if needed, that's a separate concept page.
 
 ## Test evidence
 
-Both scripts smoke-tested 2026-05-25 on disposable target dirs (env-overridden `CLAUDE_SKILLS_DIR`). Pre-populated with 2 fake stale dirs, ran full install + prune, verified: stale dirs removed, all real skills installed, retired `using-synology-ops` absent. No automated test fixture in the repo — install scripts are wrapper-style, smoke-test evidence in the `6cf0e98` commit body suffices under the `[skip-tdd: wrapper]` carve-out.
+All four scripts smoke-tested 2026-05-25.
+
+**Install side** — disposable target dirs (env-overridden `CLAUDE_SKILLS_DIR`). Pre-populated with 2 fake stale dirs, ran full install + prune, verified: stale dirs removed, all real skills installed, retired `using-synology-ops` absent.
+
+**Build side** — fake `dist/fake-stale.skill` + `dist/another-stale.skill` files created directly in the real `dist/`. Ran `build.sh --prune fake-stale-sh` (positional arg triggers a no-op build via skip path; prune runs at the end) and `build.ps1 -Names fake-stale-ps -Prune`. Both removed the fakes, left real archives like `caveman.skill` untouched.
+
+No automated test fixture in the repo — install / build scripts are wrapper-style, smoke-test evidence in the respective commit bodies suffices under the `[skip-tdd: wrapper]` carve-out.
 
 `[archive-roundtrip-test]` (still ⚪ on the board) is a candidate place to add a real fixture once it lands.