Harden `run-tests` signal and reaper guidance

Prevent unsafe harness guidance from sending SIGUSR1 before
`stackscope` installs its handler or using shared-memory scans on
unsupported platforms.

Deats,
- verify `stackscope` in the pytest env before signaling.
- target pytest by default and require a separately verified handler
  before signaling subactors.
- keep `tractor-reap --shm*` guidance to Linux and FreeBSD.
- pin staged Prompt-IO provenance to `eb3c99c9` and record the
  PR #510 review direction.

Prompt-IO: ai/prompt-io/opencode/20260822T022526Z_5562fd9a_prompt_io.md

(this patch was generated in some part by `opencode` using
`gpt-5.6-sol` (`openai`))

.claude/skills/run-tests/test-harness-reference.md

@@ -144,10 +144,17 @@ Trio-safe timeout replacement.
 For live task-tree diagnosis:
 
 ```text
+uv run --frozen --no-sync python -c 'import stackscope'
 uv run --frozen --no-sync pytest <node> --enable-stackscope --capture=sys
-kill -USR1 <pytest-or-subactor-pid>
+kill -USR1 <pytest-pid>
 ```
 
+The import check and pytest command must use the same environment. Do not send
+SIGUSR1 if the import fails or setup warns that stackscope or SIGUSR1 is
+unavailable: without the installed handler, SIGUSR1 normally terminates the
+target process. Signal a subactor only after separately confirming that it
+installed the same handler.
+
 Stackscope appends dumps to `/tmp/tractor-stackscope-<pid>.log`, including when
 pytest capture hides terminal output. SIGUSR1 stackscope is unavailable on
 Windows and degrades to a no-op there.
@@ -173,6 +180,9 @@ against the same UDS bindspace.
 
 Use the CLI in inspection-only mode first:
 
+`--shm` and `--shm-only` are Linux/FreeBSD-only and raise
+`NotImplementedError` elsewhere. On other platforms, use the UDS-only command.
+
 ```text
 uv run --frozen --no-sync scripts/tractor-reap -n
 uv run --frozen --no-sync scripts/tractor-reap --parent <pytest-pid> -n

ai/prompt-io/opencode/20260822T022526Z_5562fd9a_prompt_io.md

@@ -3,7 +3,7 @@ model: openai/gpt-5.6-sol
 service: opencode
 session: 76c5d31c-5a2f-4503-9b16-410ee7f4fab3
 timestamp: 2026-08-22T02:25:26Z
-git_ref: 5562fd9a
+git_ref: eb3c99c9
 scope: config
 substantive: true
 raw_file: 20260822T022526Z_5562fd9a_prompt_io.raw.md
@@ -39,4 +39,6 @@ Narrowed `.gitignore` to the three managed `run-tests` deployment blocks.
 The human required an existing-work scan after duplicate implementation
 was discovered, approved correcting the landing branch during PR #481
 review, and directed removal or reconciliation of unrelated ignore rules.
-No direct source-line edits were made by the human.
+During PR #510 review, the human required the stackscope and shared-memory
+safety clarifications and immutable provenance pointers before landing. No
+direct source-line edits were made by the human.

ai/prompt-io/opencode/20260822T022526Z_5562fd9a_prompt_io.raw.md

@@ -2,14 +2,14 @@
 model: openai/gpt-5.6-sol
 service: opencode
 timestamp: 2026-08-22T02:25:26Z
-git_ref: 5562fd9a
+git_ref: eb3c99c9
-diff_cmd: git diff HEAD~1..HEAD
+diff_cmd: git diff eb3c99c9^..eb3c99c9
 ---
 
 Audit Tractor's repository-wide `ai.skillz` state and correct the
 `wkt/ai_skillz_run_tests_landing` migration before landing it.
 
-> `git diff HEAD~1..HEAD -- .claude/skills/run-tests/test-harness-reference.md`
+> `git diff eb3c99c9^..eb3c99c9 -- .claude/skills/run-tests/test-harness-reference.md`
 
 The harness reference is corrected against current Tractor behavior:
 environment overrides, CI/platform matrices, randomized registry
@@ -17,7 +17,7 @@ addresses, interactive debugger capture, timeout mechanisms, stackscope
 artifacts, reaper platform limits, dry-run expansion, and exact
 change-to-test mappings.
 
-> `git diff HEAD~1..HEAD -- .gitignore`
+> `git diff eb3c99c9^..eb3c99c9 -- .gitignore`
 
 Unrelated Taken and broad OpenCode command ignore additions are removed;
 only the managed canonical `run-tests` link and command blocks remain in