Release v0.20.0: terminal workflow, first-run setup, background tasks & native modules

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Release v0.20.0: the first stable release since v0.18.0. This is the authoritative release PR; it carries the whole v0.19/v0.20 line to master in one merge.

What it contains

One linear, cumulative line from v0.18.0 (9ec06768) to the release commit:

Stage	Head	Record
v0.19.0 candidate (never published separately)	9e85a910	#390, stack #348 → #357 → #358 → #360 → #361 → #368, sibling #367
v0.19 physical-QA stabilization	1aeb05c0	#395
v0.20.0 candidate	c71a4284	this PR's original head; module/workspace/background/agent PRs per #385
v0.20 physical-QA stabilization	891ee583	#396
Post-v0.20 slices 1–3 (timeline, Queue settings, summaries, folding, paste choices, output selection and Actions)	82533639	integration/post-v020-slice3, stabilize/post-v020-slice3
First-run starting point, guided Setup, Tools flow, update notice, color editor	a469ba8e	feature/post-v020-preferences-onboarding
Release metadata	d5f8b82b, efcaa9d0	CHANGELOG 0.20.0, README/ROADMAP/AGENTS status, physical QA record; diff-check hygiene

release/v0.20.0 was fast-forwarded to the release commit (no force push); every earlier head above is an ancestor. Nothing outside this line is included: /track (#394), the scroll research, website and docs branches stay separate.

Verification (release commit efcaa9d0)

• PR CI 39083776674: pass, every job.
• Dispatch CI 39083774326 (Ubuntu/macOS shards, Node 22, Quality, Fedora, both timing smokes): attempt 1 failed in Node 22 only, shellAdapters Fish lifecycle teardown ENOTEMPTY on .local/share/fish/generated_completions (Fish's detached fish_update_completions writes after the shell exits; test fixture race, same code passed at a469ba8e). Attempt 2 of the failed jobs: pass. The failed attempt is kept, not relabelled.
• Local canonical verify:release (macOS arm64, Node 26.10.0, T7 scratch, 4 workers): build pass; suite 3047/3051 with 2 timing failures (clipboardRead fake-tool timeout, contextKernel cold-start allowance) at load average 7. Three further full-suite runs on the same commit (load average 10–13): 3048, 3049 with 0 failures, 3048; each non-clean run failed one different timing-bound case (live bash copy navigation 17.8 s, caret trail 410 ms), and no case failed twice. Benchmark typecheck, timing smoke and git diff --check: pass. No production change was made for any of these.
• Physical QA: partial. The maintainer accepted the first-run starting point, guided Setup and Tools flow on macOS; formal checklist passes were not recorded item by item, and no physical Linux/Omarchy/WSL pass was made. See docs/testing/v020-physical-qa.md and CHANGELOG → 0.20.0 → Known limitations.

Package version: 0.20.0. The tag v0.20.0 is created on the resulting master merge commit after its tree is checked against the verified release commit.