feat(boards): ✨ implement azdo boards work-item update command

[tmeckel]

Description

Implement azdo boards work-item update per #270, mirroring az boards work-item update. Updates one or more fields of an existing work item by ID via a JSON Patch document (Work Items - Update, REST 7.1).

Key decisions from the issue spec

• Project-scoped target [ORG:]PROJECT/ID parsed with util.ParseProjectTargetWithDefaultOrganization; positive-integer ID validation before any SDK call.
• Preflight ownership check: fetches the work item with GetWorkItem (fields limited to System.TeamProject) and verifies the project matches before updating. Mismatch/missing project is an error and never calls UpdateWorkItem.
• Canonical patch-doc order: Title, Description, AssignedTo, State, AreaPath, IterationPath, Reason, then raw --fields ops (split on first =) in user-given order. Tests assert this order.
• Description input modes: inline --description, repeatable --description-file (- reads stdin, concatenated with \n, 1 MB cap, binary/UTF-8 validation), and --description-editor (opens configured editor: AZDO_EDITOR -> config editor key via config.DetermineEditor -> $VISUAL -> $EDITOR -> vi/notepad; # lines stripped, empty result errors). Precedence editor > file > inline with stderr warning — shared helper in internal/cmd/boards/workitem/shared/description.go for reuse by feat: Implement azdo boards work-item create command #203.
• --discussion appends a comment via wit.AddComment (v7 AddCommentArgs.Request *CommentCreate); negative mock assertion when not set.
• Stderr warning when --bypass-rules/--suppress-notifications is set; --validate-only, --expand, --open supported.
• JSON output passes the raw *workitemtracking.WorkItem to the exporter (no view struct); default output renders the 7-column list row (ID, TYPE, STATE, TITLE, ASSIGNED TO, AREA, ITERATION).
• Uses the vendored workitemtracking.Client.UpdateWorkItem; no new mocks needed.
• Promoted fieldString/fieldIdentityDisplay from list into shared FieldString/FieldIdentityDisplay (spec: shared fields.go), shrinking list.go.

Files changed

• internal/cmd/boards/workitem/update/update.go — new command (alias u)
• internal/cmd/boards/workitem/update/update_test.go — tests covering patch-doc order, custom fields, discussion side-effect, project mismatch, ID validation, description modes, precedence warnings, JSON and table output
• internal/cmd/boards/workitem/shared/description.go, shared/fields.go (+ refactor of list.go/list_test.go) — shared helpers
• internal/cmd/boards/workitem/workitem.go — command registration
• docs/ — regenerated CLI docs

Verification

• go test ./... — all tests pass (175 in work-item packages)
• golangci-lint run ./internal/cmd/boards/workitem/... — clean

Closes #270