docs: row 211 R5 records for the busy check fixes (#1709, #426)

- BUILD.md: an R5 section covering N1 to N3, the fixes, the tests, the
  RED and GREEN counts, the 57-mutant table, the 13-shape live probe,
  the false BUSY and miss checks, and the known limits after R5. N3
  corrects the R4 bind-mount note: a same-filesystem bind mount keeps
  st_dev, so a detector needs mount metadata such as
  /proc/self/mountinfo. No code changed for N3. Earlier sections are
  untouched.
- docs/TOOLS.md: the busy check's flat form, span rule, exit 2 on text
  it can't classify, the literal listing, and 153 cases.
- BUILD-LOG.md and docs/SESSIONS.md: one appended entry each.

docs/SESSIONS.md

@@ -1882,3 +1882,4 @@ are never rewritten or removed; corrections are new entries.
 
 2026-11-02 | Sage implementation sub-agent (Claude Opus 5.5, child of T3 c5c1e8ba) | row 211 (#1709/#426) R3 from the GPT review, on t3/row211-push-guard-r3-fix over 09a847eda (picks 5734c1a80, c8f2cb70c) | F1-F5 and the object-name length check fixed with tests; suite RED 67/18, GREEN 85/0; 21 of 21 mutants killed; one local commit, not pushed
 2026-11-02 | Sage implementation sub-agent (Claude Opus 5.5, child of T3 c5c1e8ba) | row 211 (#1709/#426) R4 from the GPT review of R3, on t3/row211-push-guard-r4-fix over 09a847eda (picks bcd74ec33, 3a121885c, 7d03f212d) | R1-R3, runner exit status and installer mktemp fixed with tests; suite RED 90/14, GREEN 105/0; 30 of 30 mutants killed, runner exit 0; one local commit, not pushed
+2026-11-02 | Sage implementation sub-agent (Claude Opus 5.5, child of T3 c5c1e8ba) | row 211 (#1709/#426) R5 from the GPT review of R4, on t3/row211-push-guard-r5-fix over 09a847eda (from fd1aa8cc8) | N1-N3 fixed (literal ls listing under LC_ALL=C, flat form and awk word walk for sourced tests, bind-mount note corrected) with tests; suite RED 119/34, GREEN 153/0; 57 of 57 mutants killed, runner exit 0; 13 of 13 live shapes BUSY; two local commits, not pushed

docs/TOOLS.md

@@ -166,7 +166,7 @@ resolved through the CLI.
 | `scripts/test-conductor.sh` | Auto-apply selftests (sandboxed) | 17 cases |
 | `scripts/test-auth.sh` | Auth checkpoint selftests (no Docker) | 13 cases |
 | `scripts/test-land-work.sh` | Land-work selftests (sandboxed, fake Docker) | 84 cases |
-| `scripts/test-git-guard.sh` | Push guard, its installer and `slot-busy.sh` (scratch bare remote, fake `/proc`) | 105 cases |
+| `scripts/test-git-guard.sh` | Push guard, its installer and `slot-busy.sh` (scratch bare remote, fake `/proc`) | 153 cases |
 | `scripts/install-git-hooks.sh [--repo DIR] [--force-install]` | Install the push guard as `pre-push` in the common `.git/hooks` | Refuses a set `core.hooksPath`, a symlinked hooks directory or a different installed hook; exit 0 ok · 1 failed · 2 refused · 4 usage |
 | `scripts/slot-busy.sh` | Busy check: one `BUSY` line per gate or test process, or `CLEAR` | Exit 0 CLEAR · 1 BUSY · 2 error, including a process or an ancestor it can't read that isn't confirmed gone; chain on the exit code |
 | `scripts/gitea-api.sh <METHOD> <path> [body]` | Gitea API helper; reads the JSON credential file or a raw per-seat token file | Token never on argv/stdout |
@@ -199,11 +199,20 @@ Never `bash scripts/slot-busy.sh | tail -1 && gate`: `tail` exits 0, so the
 gate runs while BUSY. It matches `node ... --test`, a `*gate*.sh`, `mutant`, a
 `test-*.sh` and `docker compose` command lines, skips itself, its ancestors
 and agent shell-snapshot wrappers (a shell whose `-c` text starts by sourcing
-`.../shell-snapshots/snapshot-*.sh`). A wrapper whose text also sources
-(`source` or `.`) a gate, test or mutant script is BUSY, because that script
-runs in the wrapper's own shell. It reads only `/proc/<pid>/cmdline`, the cwd
-link, its own and its ancestors' `status` and, after a failed read, the proc
-root's listing. A process or an ancestor it can't read makes it exit 2, never
+`.../shell-snapshots/snapshot-*.sh`). Each rule runs on the command line as it
+is and on a flat form with the quotes and backslashes removed and every run of
+blanks, newlines and the operators `; & | ( ) { } < >` and backtick turned into
+one space, so quoting and spacing don't hide a match. A wrapper is BUSY when
+any word after a `source` or `.`, up to the next `;`, `&`, `|` or newline
+outside parentheses and backticks, names a gate, test or mutant script,
+because that script runs in the wrapper's own shell. Quoted text, an `eval`
+string for one, is read the same way. That rule can call a wrapper BUSY that
+runs no test; row 211's BUILD.md (R5) lists the cases. A command line it can't
+classify, quoting more than 32 levels deep for one, makes it exit 2. It reads
+only `/proc/<pid>/cmdline`, the cwd link, its own and its ancestors' `status`
+and, after a failed read, the proc root's listing, which it takes from GNU
+`ls --quoting-style=literal` under `LC_ALL=C` so the caller's environment
+can't change it. A process or an ancestor it can't read makes it exit 2, never
 CLEAR, unless a listing of the proc root succeeds and no longer has that pid.
 
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