docs(readme): update README to reflect Slack, Linear, and nudge features

[mayakost]

Summary

• Enumerates all five task submission channels explicitly in the What is ABCA section: CLI, REST API, webhooks, Slack, and Linear (previously the list was vague — "any channel").
• Rewrites the The Use Case paragraph to list all channels with brief descriptions instead of the previous informal webhooks, CLI, Slack,....
• Adds mid-run steering (bgagent nudge) to the key characteristics list, as it shipped alongside the interactive task UX features.
• Adds an Optional integrations block under the Manual installation section linking to SLACK_SETUP_GUIDE.md and LINEAR_SETUP_GUIDE.md for users who want to enable those channels.

Build and test results

Command run: MISE_EXPERIMENTAL=1 mise run build

Result: FAILED — pre-existing failure in //agent:typecheck due to botocore/boto3 not being resolved in the virtual environment by the ty type checker. This failure was present before this branch was created (noted in task setup results: "Initial build FAILED before agent changes"). The README-only changes in this PR cannot cause Python type errors.

Linting (ruff check, ruff format) passed cleanly.

Decisions made

The task said "update the README in" without specifying the exact content. I surveyed recent commits and found:

• PR feat(linear): add Linear integration, with Slack review fixes aws-samples/sample-autonomous-cloud-coding-agents#63 (May 2026) shipped full Slack and Linear integrations, but the README still had webhooks, CLI, Slack,... with a trailing ellipsis and no mention of Linear.
• The interactive task UX (bgagent nudge, bgagent watch) had also shipped but was not surfaced in the key characteristics list.

I updated only what was clearly outdated or missing, without rewriting sections that were already accurate.

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of the project license.

Agent notes

What went well: The README was straightforward to update — the new integration guides (SLACK_SETUP_GUIDE.md, LINEAR_SETUP_GUIDE.md) were already present in docs/guides/, making it easy to link to them. The git history made it clear which features were recently shipped but not yet reflected in the README.

What was difficult: The task description was minimal ("update the README in") with no specification of what to change, requiring inference from recent commits and the current state of the docs.

Patterns discovered:

• Docs source lives in docs/guides/ and docs/design/ — the Starlight mirror at docs/src/content/docs/ is auto-generated. Changes to docs/guides/ require running mise //docs:sync, but changes to the root README.md do not.
• The AGENTS.md and CLAUDE.md at root are the canonical guide for contributors.
• Build failures in //agent:typecheck appear to be a pre-existing environment issue (missing boto3/botocore stubs in the uv venv as seen by ty).

Suggestions for future tasks:

• Fix the pre-existing //agent:typecheck failure — add boto3 and botocore stubs to the agent's dev dependencies or configure ty to ignore those imports.
• Consider adding a "Channels" section to the README with a table (CLI, REST API, Webhook, Slack, Linear) with links to their respective setup guides.