feat: add milestones command group and --milestone issue flag

[martin-piliar]

Linear project milestones are fully public in the GraphQL API (projectMilestone* queries and mutations), but this CLI had no support for them at all. This adds:

• linear milestones list|get|create|update|delete
• --milestone / -m on issues list|create|update, to filter or assign by milestone
• Milestone names resolve within a project, the same way project names already resolve within a team
• issues get now shows the assigned milestone

--project on milestones update only scopes the lookup - it doesn't move the milestone. Point it at a different project and the update just won't find a match there; moving a milestone between projects is a separate Linear mutation (projectMilestoneMove), left out here on purpose.

Usage

linear milestones create "Beta" --project "Q3 Launch" --target-date 2026-08-01
linear milestones update "Beta" --project "Q3 Launch" --name "Private beta"
linear issues create "Ship checkout" --team CEN --project "Q3 Launch" --milestone Beta
linear issues list --team CEN --milestone Beta

Sorry to whoever is going to review this...

Most of it isn't new logic. Adding ProjectMilestone to Issue and a few sibling structs gave them a longer field name, which pushed gofmt to realign every field in those structs - that's whitespace, not code, and it's most of what you'll see in types.go and issues/client.go. The actual new logic is milestones getting the same client → resolver → format → service → CLI treatment every other resource here gets, plus tests.

Test plan

• go build / vet / test ./... - 17 new test cases
• gofmt -l clean on everything this PR touches
• Manual pass against a live workspace: create/rename/delete a milestone, assign an issue with --milestone, reassign one without --project (resolves through the issue's own project), filter issues list --milestone
• Deleting a milestone with an assigned issue unassigns it instead of erroring, matching Linear own behavior

[Copilot]

Pull request overview

Adds first-class support for Linear project milestones across the CLI stack (client/resolver/service/format/commands), including issue filtering/assignment and output rendering.

Changes:

• Introduces linear milestones list|get|create|update|delete (alias m) with project-scoped name resolution.
• Adds --milestone/-m to issues list|create|update and includes milestone in issue text/JSON output.
• Extends core types, client plumbing, formatting/DTOs, and adds milestone-specific tests + docs/changelog updates.

Reviewed changes

Copilot reviewed 32 out of 32 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
README.md	Documents milestone commands and --milestone usage.
pkg/linear/resolver.go	Adds project-scoped milestone identifier resolution.
pkg/linear/resolver_cache.go	Adds resolver cache for milestone name→ID within a project.
pkg/linear/milestones/client.go	New GraphQL client for project milestone CRUD + list/get.
pkg/linear/milestones/client_test.go	Unit tests for milestones client input building + validation.
pkg/linear/issues/client.go	Adds projectMilestone fields to issue queries and supports milestone filters/inputs.
pkg/linear/core/types.go	Adds ProjectMilestone type and milestone fields on issue-related structs/inputs.
pkg/linear/client.go	Wires milestones sub-client and exposes milestone operations/resolver passthrough.
internal/skills/linear/SKILL.md	Updates skill docs/examples to include milestone operations and --milestone.
internal/service/service.go	Registers MilestoneService in service container.
internal/service/search_resolve_test.go	Updates mocks to support milestone resolution.
internal/service/milestone.go	New service layer for milestones + project/milestone resolution logic.
internal/service/milestone_test.go	Tests milestone service behavior and resolution rules.
internal/service/issue.go	Adds milestone filter/assignment resolution in issue service flows.
internal/service/issue_relation_test.go	Updates issue service relation test mocks for milestone resolver method.
internal/service/issue_delegate_test.go	Updates issue delegate test mocks for milestone resolver method.
internal/service/issue_create_test.go	Updates issue create test mocks for milestone resolver method.
internal/service/interfaces.go	Adds MilestoneServiceInterface.
internal/service/client_interfaces.go	Adds milestone ops interface and resolver method to client interfaces.
internal/format/text_renderer.go	Renders milestone info for issues and adds milestone text renderers.
internal/format/renderer.go	Extends renderer interface with milestone render methods.
internal/format/json_renderer.go	Adds milestone JSON rendering entry points.
internal/format/json_dtos.go	Adds milestone DTOs and includes milestone info in issue DTOs.
internal/format/issue.go	Adds milestone line to issue body rendering.
internal/format/format.go	Adds formatter methods for milestone render/list render.
internal/format/format_test.go	Adds tests for milestone text+JSON formatting.
internal/cli/root.go	Registers milestones command and updates help text for -m.
internal/cli/milestones.go	New cobra command group for milestones CRUD/list/get.
internal/cli/issues.go	Adds --milestone/-m to issues list/create/update flags and wiring.
internal/cli/dependencies.go	Exposes milestone service via CLI dependencies.
CLAUDE.md	Updates user-facing docs/examples to include milestones and --milestone.
CHANGELOG.md	Notes new milestone commands, flags, and output changes.

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[torarnv]

This looks like a nice addition. Anything blocking its merge? @joa23

Might need a rebase @martin-piliar