docs: mark the 0.12.0 entry as a breaking release

The generated 0.12.0 section listed the work only under Features and Bug
Fixes, with no breaking-changes block, so a reader upgrading from 0.11.x
had nothing telling them a public parameter disappeared.

Add the `⚠ BREAKING CHANGES` section, matching the style the 0.11.0 entry
already uses, and record the migration: `extra_let_bindings` is gone from
`Session.enter_environment`, `Session.exit_environment` and
`Session.run_task`, and step-scope EXPR `let` values now travel through
`resolved_symtab`.

Also annotate the "deliver step-scope let bindings to run_task" feature
entry. Read on its own it advertises `extra_let_bindings` on `run_task`, a
parameter that does not exist in 0.12.0: it was added and removed inside
this same release. The note points at the breaking section instead.

The template does render a breaking section from
`release["elements"]["breaking"]`, but the `BREAKING CHANGE:` footer for
this release sits on a `chore:` commit (f05bf28) and no `chore` entries
reached the rendered output, so nothing was emitted. Worth a follow-up if
we want this to generate on its own next time.

Signed-off-by: David Leong <116610336+leongdl@users.noreply.github.com>

Author: leongdl

CHANGELOG.md

@@ -1,9 +1,13 @@
 ## 0.12.0 (2026-08-26)
 
 
+### ⚠ BREAKING CHANGES
+* `extra_let_bindings` is removed from `Session.enter_environment`, `Session.exit_environment` and `Session.run_task` (#357). The parameter was public in 0.11.0. Callers must deliver step-scope EXPR `let` values through the new `resolved_symtab` parameter — the resolved symbol table the service already produces per step and per environment — instead. `step_name` is unchanged on both methods. ([`457a364`](https://github.com/OpenJobDescription/openjd-sessions-for-python/commit/457a364371cb44bf5a6253e612a451e5678621d3))
+
+
 ### Features
 * accept a resolved symbol table on the v0 session (#357) ([`457a364`](https://github.com/OpenJobDescription/openjd-sessions-for-python/commit/457a364371cb44bf5a6253e612a451e5678621d3))
-* deliver step-scope let bindings to run_task ([`457a364`](https://github.com/OpenJobDescription/openjd-sessions-for-python/commit/457a364371cb44bf5a6253e612a451e5678621d3))
+* deliver step-scope let bindings to run_task — added `extra_let_bindings` to `run_task`, then superseded within this same release; see BREAKING CHANGES above ([`457a364`](https://github.com/OpenJobDescription/openjd-sessions-for-python/commit/457a364371cb44bf5a6253e612a451e5678621d3))
 * Accept resolved symbol table on v0 session ([`457a364`](https://github.com/OpenJobDescription/openjd-sessions-for-python/commit/457a364371cb44bf5a6253e612a451e5678621d3))
 
 ### Bug Fixes