fleetd #148 (point 2): drop .envrc from the default parityOverlay

.env is data; .envrc is executable shell that direnv runs on every cd, so
copying it into a worker moves behaviour, not just values. The default
parityOverlay is now [.env] only. The knob is unchanged: an operator who
wants .envrc copied can still write parityOverlay: [.env, .envrc]
explicitly.

Updates FleetConfig's default and javadoc, fleetd.example.yaml's two
mentions of the default, and the FleetConfigTest coverage: renamed the
default test, added parityOverlayExplicitEnvrcOptInStillWorks to prove
the .envrc opt-in escape hatch still works, and fixed
WorktreeSessionManagerTest#worktreeAcquireRunsParityOverlayWithProfileDefaults
which also hardcoded the old default.

fleetd/fleetd.example.yaml

@@ -172,7 +172,11 @@ herdrSocket: ~/.config/herdr/herdr.sock
 #                      skills/MCP/hooks. Omit to leave the worker on the host default.
 #   parityOverlay    → repo-relative paths copied primary→worktree so a worker in a provisioned
 #                      worktree sees the same local config (CB-301-ext). Omit for the default set:
-#                      [.env, .envrc]. (.claude/settings.local.json is NOT in the default — it
+#                      [.env] only (CB-148). .envrc is left out of the default on purpose: it is
+#                      executable shell that direnv runs on every cd, so copying it carries
+#                      behaviour into the worker, not just values, unlike .env. An operator who
+#                      wants it copied can still write parityOverlay: [.env, .envrc] explicitly.
+#                      (.claude/settings.local.json is NOT in the default — it
 #                      pre-approves IDE/tool grants a member must not hold ambiently; CB-525/CB-634.)
 #
 #                      Do NOT add .mcp.json (CB-525). A worker's tools are whatever its launcher
@@ -309,7 +313,7 @@ profiles:
     # errorPattern: "503 Service Unavailable"   # opt-in: classify a backend outage (fleetd #201/#227) — see the key doc above
     # configDir: /Users/me/.ccs/instances/gx10   # CLAUDE_CONFIG_DIR — inherit that profile's skills/MCP
     # cwd: /Users/me/src/myrepo       # pin the working dir; omit to inherit the primary's
-    # parityOverlay: [".env", ".envrc"]   # the default; never add .mcp.json or .claude/settings.local.json — see above
+    # parityOverlay: [".env"]   # the default; add ".envrc" explicitly if you want it copied too (CB-148) — never add .mcp.json or .claude/settings.local.json — see above
     # ideMcpUrl: http://127.0.0.1:29170/index-mcp/streamable-http   # opt-in (CB-634): IDE code intelligence, pinned to the worktree
     # ideProjectDir: fleetd          # CB-634: module dir the IDE opens + the overlay pins (this repo's pom is in fleetd/)
     # ideOpenCommand: env DISPLAY=:10.0 idea {dir}   # CB-634 auto-open: opens {dir} in the IDE at spawn; omit to open by hand