feat(workflows): expose {{ context.run_id }} template variable

[doquanghuy]

Description

Closes #2590.

Surfaces the engine-assigned run id (the same 8-character hex
string Spec Kit prints as Run ID: at the end of
workflow run) as a workflow template variable so YAML authors
can reference it from shell run:, command input.args:,
switch expression:, and any other field that already evaluates
{{ ... }} templates.

This is shape A from the issue ({{ context.run_id }}) —
the most discoverable option and consistent with the existing
inputs.* / steps.X.output.* naming.

Why

The run id is the natural join key between a Spec Kit workflow
run and downstream artifacts, telemetry, or per-run scratch
state. Today the operator sees it in stdout but workflows
themselves cannot reference it — there was no way to stamp a
log line, name a scratch directory, or tag an artifact with the
same id Spec Kit assigned.

The three use cases from the issue:

1. Telemetry / observability — stamp logs and events with
   the run id so external systems can join workflow runs to
   downstream artifacts.
2. Per-run scratch / isolation — interactive operator
   commands that need their own state directory under
   /tmp/run-<id>/.
3. Run-id in artifact metadata — stable join key from
   artifact back to the producing run.

Canonical usage

# Stamp telemetry events with the run id for cross-system join.
- id: emit-event
  type: shell
  run: 'echo "{\"run_id\":\"{{ context.run_id }}\",\"event\":\"started\"}" >> events.jsonl'

# Per-run scratch directory.
- id: prep-scratch
  type: shell
  run: 'mkdir -p /tmp/run-{{ context.run_id }}'

# Pass run id into a command for artifact metadata.
- id: tag-artifact
  command: speckit.specify
  input:
    args: "{{ context.run_id }}"

Implementation

StepContext.run_id is already populated by WorkflowEngine
in both execute() and resume(). The only gap was the
template namespace builder.

_build_namespace (in workflows/expressions.py) now adds a
context key alongside the existing inputs, steps, item,
and fan_in namespaces:

ns["context"] = {"run_id": run_id}

The value is always present (even outside a run) and falls back
to an empty string when no run is active. Workflows referencing
{{ context.run_id }} therefore never error — a hard
requirement from the issue's acceptance criteria for dry-run,
validation, and ad-hoc evaluator usage.

Default behaviour preserved

Workflows that do not reference {{ context.run_id }} are
byte-equivalent to before this change. The context namespace
is added unconditionally to keep template resolution
branch-free, but its presence has no observable effect when
nothing references it.

Testing

• Tested locally with uv run specify --help
• Ran existing tests with uv sync && uv run pytest
  → 2967 passed, 35 skipped (was 2960 before; +7 new
  tests added in this PR).
• Tested with a sample workflow: ran a shell step with
  run: 'echo "RUN_ID={{ context.run_id }}"' and confirmed
  the captured stdout matches the Run ID: line Spec Kit
  prints at the end of workflow run. Re-ran without the
  template reference and the workflow behaved identically
  to pre-PR.

New test coverage

TestExpressions (unit-level):

Test	What it locks
test_context_run_id_resolves	Direct lookup against StepContext(run_id=...).
test_context_run_id_defaults_to_empty_when_unset	Graceful default outside a run context (no error).
test_context_run_id_string_interpolation	Mixed template like "RUN_ID={{ context.run_id }}".

TestContextRunId (end-to-end), covering the three step types the issue's acceptance criteria called out:

Test	What it locks
test_shell_run_resolves_run_id	run: field substitution, verified via captured stdout.
test_command_input_args_resolves_run_id	input.args: resolution, captured in step output even when CLI dispatch is unavailable (the artifact-metadata use case).
test_switch_expression_matches_on_run_id	Switch matches against the resolved value, proving the run id is a first-class value in the expression engine.
test_workflow_without_context_reference_unchanged	Locks the byte-equivalent default for workflows that don't use the variable.

AI Disclosure

• I did not use AI assistance for this contribution
• I did use AI assistance (described below)

Used Claude Opus to draft the namespace change, the test suite,
the docs section, and this PR body. The shape
({{ context.run_id }} with empty-string fallback) was
proposed in the issue body; this PR implements that proposal.
Code, tests, and design decisions were human-reviewed before
submission.