fix(linear): split bgagent → abca default-label change out of PR #241 + align CLI/docs

[isadeks]

Component

Adapters (Linear) / CDK / CLI / Documentation

Problem

Two issues bundled into one:

1. Unrelated change inside PR #241. cdk/src/handlers/linear-webhook-processor.ts:34 flips DEFAULT_LABEL_FILTER from bgagent to abca. This belongs in its own PR — it has nothing to do with the screenshot pipeline.

2. The four documentation/code sites disagree on the default. Drift across:

• cdk/src/handlers/linear-webhook-processor.ts:34 (now abca on PR feat(notifications): preview-deploy screenshot pipeline (provider-agnostic) #241's branch)
• cli/src/commands/linear.ts:50 (still says bgagent)
• docs/guides/LINEAR_SETUP_GUIDE.md:89 ("Default trigger label is bgagent")
• The mapping-table doc / wherever else bgagent appears as the documented default

Spawned from krokoko's review on PR #241 (item #2, blocking).

Expected behavior

The default trigger label should be one consistent value across processor code, CLI defaults, and documentation. The change should ship in a dedicated PR with a migration note.

Current behavior

PR #241 silently flips the processor default to abca while the CLI and docs still tell users it is bgagent. Any onboarded project whose LinearProjectMappingTable row lacks an explicit label_filter would silently switch trigger label on deploy.

Proposed solution

Open a separate PR (against main, not stacked on PR #241) that:

1. Reverts the DEFAULT_LABEL_FILTER change inside PR feat(notifications): preview-deploy screenshot pipeline (provider-agnostic) #241's branch (so PR feat(notifications): preview-deploy screenshot pipeline (provider-agnostic) #241 only has the screenshot pipeline)
2. Lands the bgagent → abca change in the new PR with all four sites aligned in a single commit
3. Includes a migration note in the PR body explaining the operator-visible change for projects relying on the default
4. Updates docs/guides/LINEAR_SETUP_GUIDE.md and any other doc claiming bgagent is the default

Acceptance criteria

• PR feat(notifications): preview-deploy screenshot pipeline (provider-agnostic) #241 branch reverts the line in linear-webhook-processor.ts that flips the default
• Separate PR aligns all four sites in one commit
• Migration note in the PR body
• No grep hit for 'bgagent' as a default-label string anywhere in code or docs after the new PR lands