docs(prd): assign PRD number 0082

CI (prd-number-check) rejects unnumbered prd-new-*.md on merge to main.
Rename docs/prds/prd-new-trusted-agent-forge-identity.md to
0082-trusted-agent-forge-identity.md (0081 is claimed by #517/#519) and
update the in-repo 'PRD prd-new-trusted-agent-forge-identity' citations to
'PRD 0082'.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/unit/test_manifest_extends.py

@@ -132,7 +132,7 @@ class TestExtendsEnvMerge(unittest.TestCase):
 class TestExtendsGitMerge(unittest.TestCase):
     """git-gate.repos merges by name, with same-name child entries
     merging field-by-field (child wins). Bottles no longer carry a user
-    identity (PRD prd-new-trusted-agent-forge-identity), so only repos
+    identity (PRD 0082), so only repos
     merging is meaningful across extends chains."""
 
     _GIT_ENTRY_A = {"url": "ssh://git@host-a/a.git", "key": {"provider": "static", "path": "/dev/null"}}
@@ -339,7 +339,7 @@ class TestExtendsEgressMerge(unittest.TestCase):
 
 class TestExtendsNoBottleUserIdentity(unittest.TestCase):
     """Bottles no longer carry a user identity (PRD
-    prd-new-trusted-agent-forge-identity): identity moved to the agent's
+    0082): identity moved to the agent's
     `author` block. `git-gate.user` on a bottle is rejected outright, and
     `git_user` is always empty across an extends chain."""