docs: replace stale bash-first framing with Python-stdlib-first

The project started life as bash scripts and got rewritten to Python
(documented in docs/research/bash-vs-python-vs-go.md). Several docs
still carried the old "bash-first" framing — misleading for anyone
reading them now (8.7k lines of Python vs. ~130 lines of bash, all
in scripts/demo*.sh).

- CLAUDE.md "What this is" + "Conventions": orchestrator is Python,
  posture is stdlib-first.
- docs/prds/0010-cred-proxy.md, docs/research/manifest-format-and-
  grouping.md: quoted CLAUDE.md's old wording — re-quote.
- docs/research/built-in-supervisor-design.md, landscape-containerized-
  claude.md, agent-sandbox-landscape.md, pipelock-assessment.md,
  network-egress-guard.md: drop "bash-first" claims about the project,
  keep accurate descriptions of external tools' bash usage.

Leaves untouched: bash code-fence syntax in examples, README's
literal `bash scripts/demo.sh` invocation (the demo IS bash),
Claude Code's "Bash tool" references, IVIJL/devbox bash description
(that project actually is bash), and the bash-vs-python-vs-go
research note that records the rewrite decision.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

docs/research/manifest-format-and-grouping.md

@@ -162,7 +162,7 @@ ruamel.yaml).
 
 - **New runtime dependency.** The project today uses zero
   third-party Python packages for production code; YAML parsing
-  pulls in PyYAML. (CLAUDE.md: "bash-first, low-deps by default.")
+  pulls in PyYAML. (CLAUDE.md: "Python, stdlib-first; low-deps by default.")
 - YAML's footguns: indentation sensitivity, the Norway problem
   (`country: NO` → boolean False), implicit type coercion that's
   surprised non-trivial production projects.