Refresh Kimaki context filter for 0.13

[chubes4]

Summary

• Refresh dm-context-filter for the Kimaki 0.13 prompt shape.
• Stop stripping generic --agent <current_agent> guidance; Kimaki 0.13 now falls back to the default build agent when a requested agent does not exist.
• Treat critique instructions as managed by startup flags (--no-critique) instead of filter surgery, and document the sections that remain intentionally filtered.
• Make the effective-prompt harness model the managed --no-critique service contract and refresh snapshots against Kimaki 0.13.
• Fix the mu-plugin marker rewrite helpers so multiline blocks are portable on macOS awk, and make affected tests portable across GNU/BSD stat and checksum tools.

Closes #143.

Verification

• KIMAKI_DIST_DIR="/var/folders/lr/c_cmmt7s0592m4njz99v5yb40000gn/T/opencode/kimaki-0.13-harness/node_modules/kimaki/dist" node tests/effective-prompt/run.mjs
• for t in tests/*.sh; do printf '==> %s\n' "$t"; bash "$t"; done
• Verified Kimaki 0.13 fallback in package source: dist/session-handler/agent-utils.js falls back to the default agent when a requested agent is absent.

Notes

• Issue Remove Kimaki agent-normalization compensation after 0.13 fallback #145 was read and its Kimaki 0.13 fallback clarification is incorporated here for prompt filtering. This PR does not remove the datamachine-kimaki adapter shim; it only stops using prompt filtering as compensation for the old fallback behavior.

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (GPT-5.5)
• Used for: Drafting the code/test/doc changes, refreshing snapshots, running local verification, and preparing this PR body. Chris remains responsible for review and merge.

[chubes4]

Superseded by integration PR #162, which combines the Kimaki 0.13 cleanup work and resolves the overlapping bridge/test conflicts.