docs(notes): archive superseded session-prefix note after master retarget

The rebase onto master picked up #585 (agent-loop message machine), which
removed the request-only session-prefix seam; the unified sourced-message
decision now owns the mechanism and the note's own banner declares the
seam removed. Archive the triplet and rewrite the skill-system note's
catalog mechanism against the shipped agent/step sourced injection,
keeping the archived note as the intentional historical citation.

Re-checked the other notes master added or amended (provider retry
policies, request-error retry action, observable state machine, OTel
telemetry, dependabot): their supersessions were already executed
upstream in the same PRs; no further active note qualifies.

.agents/notes/archived/manifest.json

@@ -58,6 +58,9 @@
     "feature/2026-07-07-plan-mode.i18n.yaml": "sha256:c59b6a6c218d741cdef8edf625f1d015e409a39411fa64e65200fdebb1c49394",
     "feature/2026-07-07-plan-mode.md": "sha256:7bf1bb8e826edf68f0ec919dfd4f66955b935b46b4400d7de85fac3e4663edbc",
     "feature/2026-07-07-plan-mode.zh.md": "sha256:5b08cbcd8023f26744e481386177dd0e82423e8b0032829d0dbc22a92cced0cd",
+    "feature/2026-07-07-session-prefix.i18n.yaml": "sha256:943c50f778521f1800009cebed56d9ec192fda23b5ee9025a4f2b0e4dd8bd729",
+    "feature/2026-07-07-session-prefix.md": "sha256:1cfd8e46467111b671b0bd9c2370ef07316ff9b2846fecc0cd29389686c6bbf8",
+    "feature/2026-07-07-session-prefix.zh.md": "sha256:9a15bbed3a53db84ac12e3dfb93b00bf93dabe6b9e39bebfdfc04e00ba8ff135",
     "feature/2026-07-08-repeat-tool-guard.i18n.yaml": "sha256:e4382e8d25f23d728f1869c5195723698842dfe5422443a22f21fa8ae9b2c63c",
     "feature/2026-07-08-repeat-tool-guard.md": "sha256:95df2c423624c5c56e0b3bb76ced49e91878c7e5096cfef0faf4b20203d8a2aa",
     "feature/2026-07-08-repeat-tool-guard.zh.md": "sha256:cbe63d163aa19fc5a63233b05c8f6ed9a8a0ca215cf6f337a3c1e8e48af11b28",